buildlive: Save to $workdir/images/live and link current and previous

Built images go to $workdir/images/live to keep them separate to the
other images, we also make a symlink to current and previous image.
builddisks: Save built images to $workdir/images/disk

Built images go to $workdir/images/disk to separate them from other
images such as live and compressed ones.
build: Long live the live target

Compose trees for live images. The live target includes all the
components for the runtime target plus components that are explicitely
for the former target.

This tree will be used by buildlive to produce live images.

Issue: #27
build: Pick component build ref from name

Prepare for installed tests.
build: Use correct initramfs name

ostree doesn't correctly manage kernels yet, and so it wasn't garbage
collecting the old one.