This patch completely removes the Raspberry Pi backend and the renderer.

The backend and the renderer were written to use the proprietary
DispmanX API available only on the Raspberry Pi, to demonstrate what the
tiny computer is capable of graphics wise. They were also used to
demonstrate how Wayland and Weston in particular could leverage hardware
compositing capabilities that are not OpenGL. The backend was first
added in e8de35c9, in 2012.

Since then, the major point has been proven. Over time, support for the
rpi-backend diminished, it started to deteriorate and hinder Weston
development. On May 11, I tried to ask if anyone actually cared about
the rpi-backend, but did not get any votes for keeping it:
https://lists.freedesktop.org/archives/wayland-devel/2016-May/028764.html

The rpi-backend is a good example of how using an API that is only
available for specific hardware, even more so as it is only available
with a proprietary...