In order to allow the simple preparation of base images which
can be used for LXC/nspawn machine containers, we add this
element.
Containers inherit a kernel from the host, so there is no need
to build a kernel into the image. All the element needs is a
base init system which, in this case, is systemd.
Change-Id: I45651de2aa1b19bdeee301094f0bdffdd0a3b45c
