system: support br2-external init systems

system: support br2-external init systems

Some externals may wish to provide custom init systems for tightly
integrated boot. This has been supported through the BR2_INIT_NONE,
however a downside to the BR2_INIT_NONE is it forces the custom init
system to use either skeleton-custom and roll a custom skeleton for
each target, or skeleton-init-none which isn't a complete skeleton.

Allowing br2-external to define custom BR2_INIT_* means they can now
safely 'select' the BR2_PACKAGE_SKELETON_INIT_*, and re-use any of the
skeletons in Buildroot, or one from a br2-external tree.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>