ar71xx: fix seama factory type image generation.

Currently JFFS2 end-of-filesystem marker 0xdeadc0de is included the
computation of image's MD5 checksum as part of the seama header.  But
OpenWrt will erase blocks including and after the marker thus
invalidating the checksum after the first boot.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

SVN-Revision: 42915