ar71xx: disable jffs2 by default, it confuses users since many devices have incompatible eraseblock sizes

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36679 3c298f89-4303-0410-b956-a3cf2f4a3e73