[media] vivid: add a simple framebuffer device for overlay testing

In order to test capture and output overlays a simple framebuffer
device is created. It's bare bone, but it does the job.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>