system: Move skeleton-init-* selection to init

In preparation for supporting br2-external inits, move the 'select' for
the BR2_PACKAGE_SKELETON_INIT_* to their respective init systems. This
will allow a br2-external init to 'select' which skeleton it needs as a
default skeleton.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
[yann.morin.1998@free.fr:
  - add comment to init choice, to remind why selecting skeletons is OK
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>