ar71xx: image: add EOF marker to the WNDR4300 UBI image

ar71xx: image: add EOF marker to the WNDR4300 UBI image

Append EOF markers to the END of the UBI image
file. This forces the kernel to erase all blocks
after the marker even if those blocks are not
empty.

Additionally, the resulting image can be flashed
from the original web UI now, so use '-factory'
suffix for that.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38683 3c298f89-4303-0410-b956-a3cf2f4a3e73