package/skeleton: make it a virtual package

We now have two packages that can act as a skeleton, skeleton-common,
also known as our default skeleton, and skeleton-custom.

This means that the skeleton package can be a standard virtual package
now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>