build system: have tar use $SOURCE_DATE_EPOCH for --mtime

The --mtime argument to 'tar' sets the modification time for all files within
the archive, which determines the timestamp files will get when they are
extracted. In this case, rootfs and other tarballs will get mtimes which
correspond to the last commit timestamp of the build system, as reported by
git/subversion.

This is a step towards reproducible image builds.

Signed-off-by: bryan newbold <bnewbold@robocracy.org>
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48586