gpu-viv-bin-mx6q: change output selection mechanism

Move output selection to Config.in. It easier to add new backends selection
(wayland, directfb, ...).

Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>