util/genbuild_h: work around GNU date

Confirmed to work on FreeBSD using sh from base and bash from ports.
Verified to not break M.O. on Linux.

Change-Id: I3bce724c889c7fb760b30b25e9fc0b74620e2c53
Signed-off-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-on: http://review.coreboot.org/9056
Tested-by: build bot (Jenkins)
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
1 file changed