build: deprecate fbdev backend

Rename the build option to "deprecated-backend-fbdev" so that a
previously configured build dir doesn't retain the old setting.
This is consistent with the existing "deprecated-wl-shell" option.
Make the option default to "false".

Print a warning when fbdev is force-enabled.

Signed-off-by: Simon Ser <contact@emersion.fr>
References: #581