package/skeleton-common: simplify staging install

We don't really care what's going in staging, as long as it is properly
setup for merged/non-merged /usr, especially for the lib/ directory.

So we can just copy the skeleton as-is.

This simplifies maintenance, should we ever need to tweak the layout:
we'd just have to do it once in the skeleton directory to have it
propagated to both target and staging.

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