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>

SVN-Revision: 49016