build: enable configuration of RDP backend as a default
Add RDP to the list of backends we can set as default for use when weston is launched without display/socket environment vars set.
Co-authored-by: Steve Pronovost spronovo@microsoft.com Co-authored-by: Brenton DeGeer brdegeer@microsoft.com Signed-off-by: Hideyuki Nagase hideyukn@microsoft.com Signed-off-by: Steve Pronovost spronovo@microsoft.com Signed-off-by: Brenton DeGeer brdegeer@microsoft.com