U-blox driver rework, step 2

Moved satellite info from vehicle_gps_position_s into a new uORB topic
satellite_info.
Renamed satellites_visible to satellites_used to reflect true content.
sdlog2 will log info for GPS satellites only for now.

src/modules/mavlink/mavlink_messages.cpp

@@ -548,7 +548,7 @@ protected:
 						     cm_uint16_from_m_float(gps->epv_m),
 						     gps->vel_m_s * 100.0f,
 						     _wrap_2pi(gps->cog_rad) * M_RAD_TO_DEG_F * 1e2f,
-						     gps->satellites_visible);
+						     gps->satellites_used);
 		}
 	}
 };

src/modules/mavlink/mavlink_receiver.cpp

@@ -682,9 +682,8 @@ MavlinkReceiver::handle_message_hil_gps(mavlink_message_t *msg)
 	hil_gps.vel_ned_valid = true;
 	hil_gps.cog_rad = _wrap_pi(gps.cog * M_DEG_TO_RAD_F * 1e-2f);
 
-	hil_gps.timestamp_satellites = timestamp;
 	hil_gps.fix_type = gps.fix_type;
-	hil_gps.satellites_visible = gps.satellites_visible;
+	hil_gps.satellites_used = gps.satellites_visible;  //TODO: rename mavlink_hil_gps_t sats visible to used?
 
 	if (_gps_pub < 0) {
 		_gps_pub = orb_advertise(ORB_ID(vehicle_gps_position), &hil_gps);