build system: use SOURCE_DATE_EPOCH for timestamp if available

The SOURCE_DATE_EPOCH is an effort of the Reproducible Builds
organization to make timestamps/build dates in compiled tools
deterministic over several repetitive builds.

Busybox shows by default the build date timestamp which changes whenever
compiled. To have a reasonable accurate build date while staying
reproducible, it's possible to use the *date of last source
modification* rather than the current time and date.

Further information on SOURCE_DATE_EPOCH are available online [1].

This patch modifies `confdata.c` so that the content of the
SOURCE_DATE_EPOCH env variable is used as timestamp.

To be independent of different timezones between builds, whenever
SOURCE_DATE_EPOCH is defined the GMT time is used.

[1]: https://reproducible-builds.org/docs/source-date-epoch/

Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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