restore documentation on the build config language

Kconfig-language.txt was deleted in commit 4fa499a17b52b back in 2006.
Move to docs/ as suggested by Xabier Oneca:
  http://lists.busybox.net/pipermail/busybox/2014-May/080914.html
Also update references to it everywhere.

Signed-off-by: Kartik Agaram <akkartik@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 255 insertions, 0 deletions

diff --git a/docs/Kconfig-language.txt b/docs/Kconfig-language.txt
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+Introduction
+------------
+
+The configuration database is collection of configuration options
+organized in a tree structure:
+
+    +- Code maturity level options
+    |  +- Prompt for development and/or incomplete code/drivers
+    +- General setup
+    |  +- Networking support
+    |  +- System V IPC
+    |  +- BSD Process Accounting
+    |  +- Sysctl support
+    +- Loadable module support
+    |  +- Enable loadable module support
+    |     +- Set version information on all module symbols
+    |     +- Kernel module loader
+    +- ...
+
+Every entry has its own dependencies. These dependencies are used
+to determine the visible of an entry. Any child entry is only
+visible if its parent entry is also visible.
+
+Menu entries
+------------
+
+Most entries define a config option, all other entries help to organize
+them. A single configuration option is defined like this:
+
+config MODVERSIONS
+    bool "Set version information on all module symbols"
+    depends MODULES
+    help
+      Usually, modules have to be recompiled whenever you switch to a new
+      kernel.  ...
+
+Every line starts with a key word and can be followed by multiple
+arguments.  "config" starts a new config entry. The following lines
+define attributes for this config option. Attributes can be the type of
+the config option, input prompt, dependencies, help text and default
+values. A config option can be defined multiple times with the same
+name, but every definition can have only a single input prompt and the
+type must not conflict.
+
+Menu attributes
+---------------
+
+A menu entry can have a number of attributes. Not all of them are
+applicable everywhere (see syntax).
+
+- type definition: "bool"/"tristate"/"string"/"hex"/"integer"
+  Every config option must have a type. There are only two basic types:
+  tristate and string, the other types base on these two. The type
+  definition optionally accepts an input prompt, so these two examples
+  are equivalent:
+
+    bool "Networking support"
+  and
+    bool
+    prompt "Networking support"
+
+- input prompt: "prompt" <prompt> ["if" <expr>]
+  Every menu entry can have at most one prompt, which is used to display
+  to the user. Optionally dependencies only for this prompt can be added
+  with "if".
+
+- default value: "default" <symbol> ["if" <expr>]
+  A config option can have any number of default values. If multiple
+  default values are visible, only the first defined one is active.
+  Default values are not limited to the menu entry, where they are
+  defined, this means the default can be defined somewhere else or be
+  overridden by an earlier definition.
+  The default value is only assigned to the config symbol if no other
+  value was set by the user (via the input prompt above). If an input
+  prompt is visible the default value is presented to the user and can
+  be overridden by him.
+  Optionally dependencies only for this default value can be added with
+  "if".
+
+- dependencies: "depends on"/"requires" <expr>
+  This defines a dependency for this menu entry. If multiple
+  dependencies are defined they are connected with '&&'. Dependencies
+  are applied to all other options within this menu entry (which also
+  accept "if" expression), so these two examples are equivalent:
+
+    bool "foo" if BAR
+    default y if BAR
+  and
+    depends on BAR
+    bool "foo"
+    default y
+
+- help text: "help"
+  This defines a help text. The end of the help text is determined by
+  the level indentation, this means it ends at the first line which has
+  a smaller indentation than the first line of the help text.
+
+
+Menu dependencies
+-----------------
+
+Dependencies define the visibility of a menu entry and can also reduce
+the input range of tristate symbols. The tristate logic used in the
+expressions uses one more state than normal boolean logic to express the
+module state. Dependency expressions have the following syntax:
+
+<expr> ::= <symbol>                             (1)
+           <symbol> '=' <symbol>                (2)
+           <symbol> '!=' <symbol>               (3)
+           '(' <expr> ')'                       (4)
+           '!' <expr>                           (5)
+           <expr> '||' <expr>                   (6)
+           <expr> '&&' <expr>                   (7)
+
+Expressions are listed in decreasing order of precedence.
+
+(1) Convert the symbol into an expression. Boolean and tristate symbols
+    are simply converted into the respective expression values. All
+    other symbol types result in 'n'.
+(2) If the values of both symbols are equal, it returns 'y',
+    otherwise 'n'.
+(3) If the values of both symbols are equal, it returns 'n',
+    otherwise 'y'.
+(4) Returns the value of the expression. Used to override precedence.
+(5) Returns the result of (2-/expr/).
+(6) Returns the result of min(/expr/, /expr/).
+(7) Returns the result of max(/expr/, /expr/).
+
+An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
+respectively for calculations). A menu entry becomes visible when it's
+expression evaluates to 'm' or 'y'.
+
+There are two type of symbols: constant and nonconstant symbols.
+Nonconstant symbols are the most common ones and are defined with the
+'config' statement. Nonconstant symbols consist entirely of alphanumeric
+characters or underscores.
+Constant symbols are only part of expressions. Constant symbols are
+always surrounded by single or double quotes. Within the quote any
+other character is allowed and the quotes can be escaped using '\'.
+
+Menu structure
+--------------
+
+The position of a menu entry in the tree is determined in two ways. First
+it can be specified explicitely:
+
+menu "Network device support"
+    depends NET
+
+config NETDEVICES
+    ...
+
+endmenu
+
+All entries within the "menu" ... "endmenu" block become a submenu of
+"Network device support". All subentries inherit the dependencies from
+the menu entry, e.g. this means the dependency "NET" is added to the
+dependency list of the config option NETDEVICES.
+
+The other way to generate the menu structure is done by analyzing the
+dependencies. If a menu entry somehow depends on the previous entry, it
+can be made a submenu of it. First the the previous (parent) symbol must
+be part of the dependency list and then one of these two condititions
+must be true:
+- the child entry must become invisible, if the parent is set to 'n'
+- the child entry must only be visible, if the parent is visible
+
+config MODULES
+    bool "Enable loadable module support"
+
+config MODVERSIONS
+    bool "Set version information on all module symbols"
+    depends MODULES
+
+comment "module support disabled"
+    depends !MODULES
+
+MODVERSIONS directly depends on MODULES, this means it's only visible if
+MODULES is different from 'n'. The comment on the other hand is always
+visible when MODULES it's visible (the (empty) dependency of MODULES is
+also part of the comment dependencies).
+
+
+Kconfig syntax
+--------------
+
+The configuration file describes a series of menu entries, where every
+line starts with a keyword (except help texts). The following keywords
+end a menu entry:
+- config
+- choice/endchoice
+- comment
+- menu/endmenu
+- if/endif
+- source
+The first four also start the definition of a menu entry.
+
+config:
+
+    "config" <symbol>
+    <config options>
+
+This defines a config symbol <symbol> and accepts any of above
+attributes as options.
+
+choices:
+
+    "choice"
+    <choice options>
+    <choice block>
+    "endchoice"
+
+This defines a choice group and accepts any of above attributes as
+options. A choice can only be of type bool or tristate, while a boolean
+choice only allows a single config entry to be selected, a tristate
+choice also allows any number of config entries to be set to 'm'. This
+can be used if multiple drivers for a single hardware exists and only a
+single driver can be compiled/loaded into the kernel, but all drivers
+can be compiled as modules.
+A choice accepts another option "optional", which allows to set the
+choice to 'n' and no entry needs to be selected.
+
+comment:
+
+    "comment" <prompt>
+    <comment options>
+
+This defines a comment which is displayed to the user during the
+configuration process and is also echoed to the output files. The only
+possible options are dependencies.
+
+menu:
+
+    "menu" <prompt>
+    <menu options>
+    <menu block>
+    "endmenu"
+
+This defines a menu block, see "Menu structure" above for more
+information. The only possible options are dependencies.
+
+if:
+
+    "if" <expr>
+    <if block>
+    "endif"
+
+This defines an if block. The dependency expression <expr> is appended
+to all enclosed menu entries.
+
+source:
+
+    "source" <prompt>
+
+This reads the specified configuration file. This file is always parsed.