package/skeleton-custom: simplify target/staging install

For target install, we don't need to check the merged-usr symlinks
any more, it was already checked before.

For staging, instead of creating directories, just copy the same
skeleton, which was already checked to be correct.

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