Fix variation in timestamps caused by timezone differences.

When building with SOURCE_DATE_EPOCH set, avoid use of mktime in
default_image.c, which converts the timestamp into localtime. This
causes variation based on timezone when building u-boot.img and
u-boot-sunxi-with-spl.bin targets.

Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Tested-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Paul Kocialkowski <contact@paulk.fr>