gps_position: convert uint64 timestamp_time -> int32 timestamp_time_relative

We need to make this timestamp relative to the main timestamp. Necessary
for replay, and saves some space.

msg/vehicle_gps_position.msg

@@ -25,7 +25,7 @@ float32 vel_d_m_s		# GPS Down velocity, (metres/sec)
 float32 cog_rad			# Course over ground (NOT heading, but direction of movement), -PI..PI, (radians) 
 bool vel_ned_valid		# True if NED velocity is valid 
 
-uint64 timestamp_time		# Time of the UTC timestamp since system start, (microseconds) 
+int32 timestamp_time_relative	# timestamp + timestamp_time_relative = Time of the UTC timestamp since system start, (microseconds)
 uint64 time_utc_usec		# Timestamp (microseconds, UTC), this is the timestamp which comes from the gps module. It might be unavailable right after cold start, indicated by a value of 0 
 
 uint8 satellites_used		# Number of satellites used 

src/drivers/gps/devices

@@ -1 +1 @@
-Subproject commit 682fca425e6f3d67ac8062dcc9b36d8bca210175
+Subproject commit a5e3e263aaaa613e007717d3ebf5ca7c597bcf51

src/drivers/gps/gps.cpp

@@ -752,10 +752,9 @@ GPS::task_main()
 					 * no valid position lock
 					 */
 
-					_report_gps_pos.timestamp_time = hrt_absolute_time();
-
 					/* reset the timestamp for data, because we have no data yet */
 					_report_gps_pos.timestamp = 0;
+					_report_gps_pos.timestamp_time_relative = 0;
 
 					/* set a massive variance */
 					_report_gps_pos.eph = 10000.0f;

src/modules/mavlink/mavlink_receiver.cpp

@@ -1781,7 +1781,7 @@ MavlinkReceiver::handle_message_hil_gps(mavlink_message_t *msg)
 	struct vehicle_gps_position_s hil_gps;
 	memset(&hil_gps, 0, sizeof(hil_gps));
 
-	hil_gps.timestamp_time = timestamp;
+	hil_gps.timestamp_time_relative = 0;
 	hil_gps.time_utc_usec = gps.time_usec;
 
 	hil_gps.timestamp = timestamp;

src/platforms/posix/drivers/gpssim/gpssim.cpp

@@ -339,7 +339,7 @@ GPSSIM::task_main()
 			//Make sure to clear any stale data in case driver is reset
 			memset(&_report_gps_pos, 0, sizeof(_report_gps_pos));
 			_report_gps_pos.timestamp = hrt_absolute_time();
-			_report_gps_pos.timestamp_time = hrt_absolute_time();
+			_report_gps_pos.timestamp_time_relative = 0;
 
 			if (!(m_pub_blocked)) {
 				if (_report_gps_pos_pub != nullptr) {