uboot-kirkwood/goflexhome: update bootargs to use new ubi style

Since switching to new ubi(kernel,squashfs,ubifs) layout, the kernel lies in a
ubi volume. Dropping the mtd parts because the mtd layout is saved in the
device-tree, which is appended to the zImage.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49016 3c298f89-4303-0410-b956-a3cf2f4a3e73