ar71xx: disable routerboard support in the generic subtarget, enable it for the nand subtarget only

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

git-svn-id: svn://svn.openwrt.org/openwrt@36879 3c298f89-4303-0410-b956-a3cf2f4a3e73