kirkwood: update support for ib62x0

The board is optimized for ubi(fs) and boots OpenWrt without changes to uboot
environment if OpenWrt's uboot package is used. In order to flash the image run
following commands in uboot shell:

nand erase 0x200000 0xfe00000
ubi part root ; ubi remove rootfs ; ubi create rootfs
tftpboot 0x800000 openwrt-kirkwood-ib62x0-rootfs.ubifs ; ubi write 0x800000 rootfs ${filesize} ; reset

Signed-off-by: Luka Perkov <luka@openwrt.org>
Tested-by: Luka Perkov <luka@openwrt.org>
CC: Imre Kaloz <kaloz@openwrt.org>

SVN-Revision: 39592