GPS Yaw: wait to fuse at yaw at least once before declaring it faulty

This fixes the cases where the yaw message from the GNSS receiver would
take more time than the vel/pos. The estimator should wait and not
immediately fall back to an other aiding source after 5sec.
If it never comes, it will never fall back, but this is ok since the
user wants to fly with GPS aiding an not with something else.

test/sensor_simulator/gps.cpp

@@ -102,7 +102,7 @@ gps_message Gps::getDefaultGpsData()
 	gps_data.lat = 473566094;
 	gps_data.lon = 85190237;
 	gps_data.alt = 422056;
-	gps_data.yaw = 0.0f;
+	gps_data.yaw = NAN;
 	gps_data.yaw_offset = 0.0f;
 	gps_data.fix_type = 3;
 	gps_data.eph = 0.5f;