Add some documentation about the DRM backend into its own man page, and
refer to it in weston(1).
Environment variable, that are reserved for backends, and currently used
only by the DRM backend, are moved to weston-drm page.
NOTE: This is a candidate for the stable branch
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
Pekka Paalanen