mavlink: fix boot complete without lockstep

Without lockstep the actual monotonic clock of the host computer is
used. Therefore, this time is likely much more than 20 seconds and the
check if the boot complete happened in time will fail immediately.

Therefore, it probably makes more sense to count the time from the first
mavlink creation.

src/modules/mavlink/mavlink_receiver.cpp

@@ -2529,7 +2529,7 @@ MavlinkReceiver::Run()
 
 	// make sure mavlink app has booted before we start processing anything (parameter sync, etc)
 	while (!_mavlink->boot_complete()) {
-		if (hrt_absolute_time() > 20_s) {
+		if (hrt_elapsed_time(&_mavlink->get_first_start_time()) > 20_s) {
 			PX4_ERR("system boot did not complete in 20 seconds");
 			_mavlink->set_boot_complete();
 		}