update/release: if *GNU* tar, use --mtime

with --mtime, files added to the archive can be set
to a static date (in this case, the unix epoch)

the one used here is derived from git commit dates,
and it is static; if not being handled in lbmk.git,
the versiondate file never changes

this is the first patch in a series of patches designed
to bring about reproducible builds in libreboot

a solution will need to be found, for non-GNU tar
implementations, because they did not have an
equivalent option according to their manpages.
for example, BSD tar implementations.

perhaps i could systematically go around changing
file dates, on each file, as a fallback behaviour?

Signed-off-by: Leah Rowe <leah@libreboot.org>

0 files changed, 0 insertions, 0 deletions