We should not check the EVPN transport protocol support for control-plane-only devices (for example, cRPD acting as EVPN RR).

I think this should be guarded with an extra feature (for example, features.evpn.cp_only) in case a device does something weird even when being only a route reflector (I wouldn't be surprised).

Thoughts @ssasso @jbemmel @leec-666 @nmodena @rickycraft ?

Based on #3219