Don't start HAP server without any devices

This ensures that when iOS reconnects, it doesn't find an empty server
and thinks all the devices have been removed, and removes them all
locally.

bridge.go

@@ -212,6 +212,11 @@ func (br *Bridge) WaitConfigured() {
 	}
 }
 
+// Return number of devices added to the bridge.
+func (br *Bridge) NumDevices() int {
+	return len(br.devices)
+}
+
 // Connects to the MQTT server.
 // Blocks until the connection is established, then auto-reconnect logic takes over
 func (br *Bridge) ConnectMQTT() error {