Makefile: generate SOURCE_DATE_EPOCH for reproducible builds

When reproducibility is requested, generate a global SOURCE_DATE_EPOCH
environment variable which contains either the date of Buildroot last
commit if running from a git repository, or the latest release date.

This means that all packages embedding build dates will appear to
have the same build date, so in case of new commit or release, all
packages will appear to have been changed, even though some of them
may not have changed in fact.

The meaning of SOURCE_DATE_EPOCH is specified by the following
specification:
  https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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