mesh_router_t::allow_router_switch field

if the BSSID is specified and this value is also set, when the router of this specified BSSID fails to be found after "fail" (mesh_attempts_t) times, the whole network is allowed to switch to another router with the same SSID. The new router might also be on a different channel. The default value is false. There is a risk that if the password is different between the new switched router and the previous one, the mesh network could be established but the root will never connect to the new switched router.