system: support br2-external skeleton packages

Today, the BR2_ROOTFS_SKELETON_CUSTOM is the only way to build a custom
skeleton. But it's limiting as users must provide a pre-built skeleton
for each target. Supporting a br2-external package allows users to build
up a skeleton and customize it with their own KConfig options.

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