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);
 		}
 	}
 };