scripts/arm-magic.sh: make the generated kernels also include $IMG_PREFIX

In r45785 ixp4xx was changed to use a common image prefix also for
kernels, but the script creating them was not updated to generate the
expected filenames.
Fix this by letting arm-magic.sh respect $IMG_PREFIX for the output
kernel file names as well.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 45803