weston: improve selection of compositor

The way the compositor was selected in Config.in was counter-intuitive,
because the fbdev backend is selected by default even if a different one
is available.

Instead, select the fbdev backend only if no other one was selected by
the user.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr: don't reorder entries, keep alphabetical sort]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>