package/skeleton: split out into skeleton-common

Move all the handling of the default skeleton into a new package,
skeleton-common.

We don't name it skeleton-default, because it will be further split
later, into a skeleton for sysv and another for systemd, with some parts
still common between the two. So just name it skeleton-common right now;
this will save us a rename later.

While we're at it, also assign to SKELETON_COMMON_TARGET_FINALIZE_HOOKS
instead of directly to the global FINALIZE_HOOKS. Therefore, we don't
need to do all of that in a condition BR2_PACKAGE_SKELETON_COMMON==y.

Note: it would be technically sound to move the skeleton files together
within a sub-directory of the skeleton-common package. However, we refer
the user to those files, from various locations (manual, packages). It
will indeed be easier for the user to find those files in
system/skeleton/ rather than in package/skeleton-common/skeleton/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout: remove the mkdir $(STAGING_DIR)/usr/include which was removed
         in skeleton.mk in master.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>