fs: remove intermediate artefacts

fs: remove intermediate artefacts

Each of the intermediate, per-rootfs target directories, as well as the
intermediate tarball, can take quite some place, and is mostly a
duplication of what's already in target/. The only delta, if any, would
be the tweaks made by the filesystem image generations, but those tweaks
are most probably only meaningful when seen as root.

We normally do not remove intermediate files, but those can be quite
large, and are not directly usable by, nor accessible to the user.
So, get rid of them once the filesystem has been generated.

This does not need to be done in fakeroot.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>