ar71xx: Fix sysupgrade when upgrading to images with small rootfs partition

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38382 3c298f89-4303-0410-b956-a3cf2f4a3e73