package/skeleton: introduce sysv, systemd and none specific skeletons

Currently, we use the same skeleton for sysv-like init systems and
systemd, even though systemd has some peculiarities that makes our
default skeleton unfit.

So, we'll need to provide different skeletons (really, only part of
it) for sysv-like and systemd. In addition, in order to support the
"no init system" (BR2_INIT_NONE) use case, we introduce a "none"
skeleton.

Introduce three new skeleton packages, aptly named skeleton-sysv,
skeleton-systemd and skeleton-none. All three are providers of the
skeleton virtual package, in lieu of the skeleton-common package,
which is now a simple dependency of all three new skeletons.

Those packages are empty for now. In followup changes:
  - sysv-specific stuff will be moved out of skeleton-common and into
    skeleton-sysv;
  - systemd-specific stuff will be added to skeleton-systemd.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout:
 - merge with the patch that enables the BR2_INIT_NONE case
 - simplify the BR2_PACKAGE_SKELETON_COMMON_ONLY select logic]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas:
 - remove the BR2_PACKAGE_SKELETON_COMMON_ONLY logic, and instead
   introduce a separate skeleton-none package for the BR2_INIT_NONE]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>