scripts/ubinize-image.sh: fix ubinize-image.sh when used with --kernel

r41359 broke support for building ubinized images which do
include a kernel volume.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41368 3c298f89-4303-0410-b956-a3cf2f4a3e73