ipq40xx: cellc_rtl30vw: fix imagebuilder generation

ipq40xx: cellc_rtl30vw: fix imagebuilder generation

The image build process was modifying the generated IMAGE_KERNEL to
append rootfs information (crc). This caused:
 - sysupgrade & factory images to contain 2 times the root.squashfs
   information due to both modifying the same IMAGE_KERNEL.
 - the generated imagebuilder to contain an erroneous IMAGE_KERNEL that
   contained references to an unexisting root.squashfs (the one from
   previous cause). The RTL30VW wasn't therefore able to boot the
   generated images as they contained checksums from non existing rootfs.

This commit makes sure to use a temporary IMAGE_KERNEL to append the
rootfs information for both factory and sysupgrade images.

Fixes: #10511
Signed-off-by: Gregory Detal <gregory.detal@tessares.net>