package/skeleton: split out into skeleton-custom

For the custom skeleton, we practicaly do nothing, except ensure it
contains the basic, required directories, and that those are properly
setup wrt. merged /usr.

Furthermore, our current skeleton is not fit for systemd, and we'll
have to split things out into various skeletons.

So, off-load the custom skeleton into its own package.

Thus, the existing skeleton package is now limited to:

  - when using our default skeleton, install and tweak it properly;

  - when using a custom skeleton, do nothing except for depending on
    the skeleton-custom package.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout: split off in a separate patch doing only this]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>