[adm8668] fix firstboot with new MTD map driver

[adm8668] fix firstboot with new MTD map driver

my-mkimage previously did not include the fs_mark of deadc0de because
upon formatting of jffs2 partition, U-Boot's CRC validation would not
allow the image to boot. This new MTD map will shrink the
kernel+rootfs+fs_mark image and recalculate the CRC, so that only the
kernel is part of the image validation. This also improves boot time,
since less is copied from FLASH->RAM. This is mostly from brcm47xx,
just with u-boot image vs TRX partitions.

Signed-off-by: Scott Nicholas <scott.nicholas@scottn.us>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@25176 3c298f89-4303-0410-b956-a3cf2f4a3e73