package/matchbox: make matchbox-wm a real package

Similarly to what we just did for matchbox-lib, we make matchbox-wm a
real package.

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