If SOURCE_DATE_EPOCH is unset (in addition to the existing "0" behaviour)
parse out the top most commit timestamp from the kernel tree to use as the
timestamp.
(From OE-Core rev: 2f0dd67a5a8d4269f5155004d532d8fa972b3223)
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>