build: introduce SOURCE_DATE_EPOCH variable

SOURCE_DATE_EPOCH is the date of the last modified file using git/svn
as date source.
See https://reproducible-builds.org/specs/source-date-epoch/



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

SVN-Revision: 48584