kirkwood: fix uimage creation for some kirkwood devices

because with the current system I'm getting fake uImages that are actually a
renamed zImage, and that's plain wrong.
This fixes bug https://bugs.lede-project.org/index.php?do=details&task_id=131
and https://bugs.lede-project.org/index.php?do=details&task_id=139



Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>