Implement a "well" defined API to configure the fbdev backend.
Following and according to discussion about libweston API

The output transform configuration is moved into weston and added to the
fbdev configuration structure.

Signed-off-by: Benoit Gschwind <gschwind@gnu-log.net>
[Pekka: squashed two patches and rebased.]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>