mediatek: convert the NAND target to UBI

Signed-off-by: John Crispin <john@phrozen.org>

mediatek: sync and patches add support for several boards

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49263 3c298f89-4303-0410-b956-a3cf2f4a3e73