rootfs.py: dont try to list installed packages for baremetal images

rootfs.py: dont try to list installed packages for baremetal images

Theres not a rootfs for baremetal images, hence we should avoid
trying to list rootfs packages for them.

This fixes an issue where some classes (e.g. license_image) rely on
rootfs functionality when included for baremetal images even if its
nonexistent

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>