Add a new backend for the Raspberry Pi.

This backend uses the DispmanX API to initialise the display, and create
an EGLSurface, so that GLESv2 rendering is shown on the "framebuffer".
No X server is involved. All compositing happens through GLESv2.

The created EGLSurface is specifically configured as buffer content
preserving, otherwise Weston wouuld show only the latest damage and
everything else was black. This may be sub-optimal, since we are not
alternating between two buffers, like the DRM backend is, and content
preserving may imply a fullscreen copy on each frame.

Page flips are not properly hooked up yet. The display update will
block, and we use a timer to call weston_output_finish_frame(), just
like the x11 backend does.

This backend handles the VT and tty just like the DRM backend does.
While VT switching works in theory, the display output seems to be
frozen while switched away from Weston. You can still switch back.

Seats and connectors cannot be explicitly specified, and multiple seats
are not expected.

Udev is used to find the input devices. Input devices are opened
directly, weston-launch is not supported at this time. You may need to
confirm that your pi user has access to input device nodes.

The Raspberry Pi backend is built by default. It can be build-tested
without the Raspberry Pi headers and libraries, because we provide stubs
in rpi-bcm-stubs.h, but such resulting binary is non-functional. If
using stubs, the backend is built but not installed.

VT and tty handling, and udev related code are pretty much copied from
the DRM backend, hence the copyrights. The rpi-bcm-stubs.h code is
copied from the headers on Raspberry Pi, including their copyright
notice, and modified.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>