Respect SOURCE_DATE_EPOCH when building FIT images.

Embedding timestamps in FIT images results in unreproducible builds
for targets that generate a fit image, such as dra7xx_evm.

This patch uses the SOURCE_DATE_EPOCH environment variable, when set,
to use specified value for the date.

Thanks to HW42 for debugging the issue and providing the patch:

  https://lists.alioth.debian.org/pipermail/reproducible-builds/Week-of-Mon-20160606/005722.html

For more information about reproducible builds and the
SOURCE_DATE_EPOCH specification:

  https://reproducible-builds.org/specs/source-date-epoch/
  https://reproducible-builds.org/



Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Simon Glass <sjg@chromium.org>