We have a list of images which are unbuildable for distro/buildtype/arch
combos. This patch renames it from SKIPPED_IMAGES to UNBUILDABLE_IMAGES
because 'skipped' images is something else in our code.
Now all unbuildable images are marked as such and so are their children.
At the end of build there is information provided about skipped images
(due to --skip-parents or --skip-existing options) and then information
about not buildable images. Both list should only contain entries
related to requested build.
Change-Id: If9b521339f564e483cba03d52e7c4eba271821a5