Newer NAND devices from MikroTik like the hAP ac3
require the kernel to be packed into UBIFS and then
ubinized.
So, since the ubinize-image.sh script can now ubinize
kernel only as well lets add a command for it.
This now allows calling ubinize-kernel in the kernel
packaging at then end.
Signed-off-by: Robert Marko <robimarko@gmail.com>