skeleton: Rename skeleton-systemd to skeleton-init-systemd

skeleton: Rename skeleton-systemd to skeleton-init-systemd

The skeletons are based on the selection of BR2_INIT_*, so add init- to
the package name to make this clearer. While skeleton-systemd is
relatively clear, skeleton-common and skeleton-none are less clear on
their relationship to BR2_INIT_*. So rename skeleton-systemd to conform
to clearer pattern.

Signed-off-by: Cam Hutchison <camh@xdna.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>