package/matchbox: make matchbox-desktop a real package

Currently, matchbox-desktop is treated as if it were an option of the
matchbox package.

But that's not true, since it is a real package all by itself, with its
own archive and its own buildsystem.

So, make it 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>