Split the cleanup_dirs function in two, i.e. cleanup of the build dir
and the image dir, and use the former to cleanup the temporary build
subdirs after their unmount, before the conversion to other disk
formats; they are not needed anyway at that point, and allows to save
disk space during the conversion phase.
Change-Id: Ie30d7e6033613d6979148423326ae7e17a7342e7