MAVLink app: Provide function for connected state

If we have not received an inbound heartbeat we are probably not connected and so user-facing output (which is even buffered) should go only out once we are connected.

src/modules/mavlink/mavlink_main.h

@@ -214,6 +214,8 @@ public:
 
 	void			set_config_link_on(bool on) { _config_link_on = on; }
 
+	bool			is_connected() { return ((_rstatus.heartbeat_time > 0) && (hrt_absolute_time() - _rstatus.heartbeat_time < 3000000)); }
+
 	bool			broadcast_enabled() { return _broadcast_mode > BROADCAST_MODE_OFF; }
 
 	/**