core/show-info: report image name of filesystems

core/show-info: report image name of filesystems

The rootfses, unlike packages, do not have stampfiles to represent
whether they are built or not. Indeed, we always rebuild the rootfs, and
there is a single step to do.

Hpwever, people (and scripts) who want to report on the build progress,
will want to know whether each rootfs has been built already or not.

Expose in show-info the name of the file that wil contain the rootfs
image.

$(1)_FINAL_IMAGE_NAME is set by the fs infrastructure, but initramfs
doesn't use the infrastructure. So to support that one as well, set
image_name to null if $(1)_FINAL_IMAGE_NAME is not set.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: eeppeliteloop@gmail.com
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>