tools/mkimage: backport SOURCE_DATE_EPOCH for reproducible builds

This pulls in Paul Kocialkowski's SOURCE_DATE_EPOCH support patch for u-boot,
which landed upstream circa July 2015. Note that this "host" u-boot repo is
only used to compile the 'mkimage' utility, and isn't used to actually compile
a bootloader for any target.

This patch could be removed if/when the host u-boot package is updated to a
contemporary version (but there doesn't seem to be any motivation/need to do
so).

Signed-off-by: bryan newbold <bnewbold@robocracy.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org> [fix portability error, refresh patches]
SVN-Revision: 48542