skeleton: Rename skeleton-common to skeleton-init-common

skeleton: Rename skeleton-common to skeleton-init-common

The skeletons are based on the selection of BR2_INIT_*, so add init- to
the package name to make this clearer. The name skeleton-common implies
that it is common to all skeletons, yet it does not apply to
skeleton-custom. It is only common to the skeleton-init-* packages, so
name it the same way.

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>