package/weston: add support for the RDP compositor

Using the RDP compositor, one can run a headless machine to serve remote
clients, using the RDP protocol.

Add an option to enable the rdp-backend.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>