add explicit non-callback contructor for nuttx/uorb subscriber to work around linker issues

src/platforms/px4_defines.h

@@ -154,7 +154,7 @@ static inline px4_param_t PX4_ROS_PARAM_SET(const char *name, float value)
 /* Subscribe and providing a function as callback (do not use directly, use PX4_SUBSCRIBE instead) */
 #define PX4_SUBSCRIBE_CBFUNC(_nodehandle, _name, _cbf, _interval) _nodehandle.subscribe<PX4_TOPIC_T(_name)>(PX4_TOPIC(_name), std::bind(&_cbf, std::placeholders::_1), _interval)
 /* Subscribe without a callback (do not use directly, use PX4_SUBSCRIBE instead) */
-#define PX4_SUBSCRIBE_NOCB(_nodehandle, _name, _interval) _nodehandle.subscribe<PX4_TOPIC_T(_name)>(PX4_TOPIC(_name), nullptr, _interval)
+#define PX4_SUBSCRIBE_NOCB(_nodehandle, _name, _interval) _nodehandle.subscribe<PX4_TOPIC_T(_name)>(PX4_TOPIC(_name), _interval)
 
 /* Parameter handle datatype */
 #include <systemlib/param/param.h>

src/platforms/px4_nodehandle.h

@@ -178,6 +178,26 @@ public:
 		return (Subscriber<M> *)sub_px4;
 	}
 
+	/**
+	 * Subscribe without callback to function
+	 * @param meta		Describes the topic which nodehande should subscribe to
+	 * @param interval	Minimal interval between data fetches from orb
+	 */
+
+	template<typename M>
+	Subscriber<M> *subscribe(const struct orb_metadata *meta,
+			      unsigned interval)
+	{
+		SubscriberUORB<M> *sub_px4 = new SubscriberUORB<M>(meta, interval, &_subs);
+
+		/* Check if this is the smallest interval so far and update _sub_min_interval */
+		if (_sub_min_interval == nullptr || _sub_min_interval->getInterval() > sub_px4->getInterval()) {
+			_sub_min_interval = sub_px4;
+		}
+
+		return (Subscriber<M> *)sub_px4;
+	}
+
 	/**
 	 * Advertise topic
 	 * @param meta		Describes the topic which is advertised

src/platforms/px4_subscriber.h

@@ -178,6 +178,21 @@ public:
 		//XXX store callback
 	{}
 
+	/**
+	 * Construct SubscriberUORB by providing orb meta data without callback
+	 * @param meta	    orb metadata for the topic which is used
+	 * @param interval	Minimal interval between calls to callback
+	 * @param list	    subscriber is added to this list
+	 */
+	SubscriberUORB(const struct orb_metadata *meta,
+		      unsigned interval,
+		      List<uORB::SubscriptionNode *> *list) :
+		Subscriber<M>(),
+		uORB::Subscription<M>(meta, interval, list),
+		_callback(nullptr)
+		//XXX store callback
+	{}
+
 	~SubscriberUORB() {};
 
 	/**