ar71xx/all0258n: sysupgrade support

U-Boot on the ALL0258N needs offset, size and md5 for kernel and rootfs to be
stored in the U-Boot environment.
If the checksums don't match during boot, a failsafe-system is booted instead.
This patch adds a board-specific sysupgrade hack for the all0258n which
calculates and updates the checksums for the U-Boot environment.

Signed-off-by: Daniel Golle <dgolle@allnet.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28700 3c298f89-4303-0410-b956-a3cf2f4a3e73