core/pkg-generic: also list files installed in images/

Some packages install nothing in target nor staging, but install images
(like the kernel vmlinux, or a bootloader boot blob...)

If we want to appropriately account for the files installed by each
package, we also need to take images/ into account.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>