mostly revert commit r22396

CFE does not boot images generated with these checksums because of
wrong checksum.

After flashing then with tftp to my Asus wl500-GPv1 the following messages
are show:
Null Rescue Flag.
Boot program checksum is invalid
Hello!! Enter Rescue Mode: (Check error)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@22418 3c298f89-4303-0410-b956-a3cf2f4a3e73