package/matchbox: make matchbox-fakekey a real package

Currently, matchbox-fakekey is missing a Kconfig entry, but is so far
solely a build-time dependency of matchbox-keyboard.

So, make it a real package, and select it from matchbox-keyboard.

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>