reproducible: fix git SOURCE_DATE_EPOCH randomness

reproducible: fix git SOURCE_DATE_EPOCH randomness

Anything that defines multiple git sources should have the largest value
taken when calculating the SOURCE_DATE_EPOCH for a package.

The previous iteration actually introduced some degree of randomness, as
it would stop on the first git repository reported by os.walk, which
does not assure any specific ordering by default.

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>