image.mk: make ubinize.cfg optional

image.mk: make ubinize.cfg optional

When using UbinizeImage with ubifs rootfs, ubinize.cfg is no longer
needed. Yet, the absance of ubinize.cfg would make the build process
abort with an error.
Fix that by checking if ubinize.cfg is present and do no not call the
"classic" ubinize image generation if it isn't.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[missing new-line before UbinizeImage added itentionally]

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43788 3c298f89-4303-0410-b956-a3cf2f4a3e73