weston: Port fbdev backend to new output handling API

This is a complete port of the fbdev backend that uses
the recently added output handling API for output
configuration.

It is required that the scale and transform values are
set using the previously added functionality.

After everything has been set, output needs to be
enabled manually using weston_output_enable().

v2:

 - Use weston_compositor_add_pending_output().
 - Bump weston_fbdev_backend_config version to 2.

Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>