package/rpi-firmware: only install images

The rpi-firmware only installs images files, so it should use
_INSTALL_IMAGES_CMDS and not _INSTALL_TARGET_CMDS.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>