bcm53xx: switch RT-N18U firmware to the UBI

According to the ticket #18152 it also uses NAND with 0x20000 erase size
and 0x800 page size.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43008 3c298f89-4303-0410-b956-a3cf2f4a3e73