fix commander: run preflight checks on GCS connection

Regression from 6dec451, leading to
preflight failures not being reported at all. Only after a failed arming
attempt the messages would be sent. And for GPS check failures, in case
they are set to optional (default), arming would be possible, but switching
to position would be rejected w/o error.

We need to run the preflight checks periodically, but this at least restores
the previous behavior.

src/modules/commander/Commander.cpp

@@ -3558,12 +3558,18 @@ void Commander::data_link_check()
 			switch (telemetry.remote_type) {
 			case telemetry_status_s::MAV_TYPE_GCS:
 
-				// Recover from data link lost
+				// Initial connection or recovery from data link lost
 				if (status.data_link_lost) {
 					if (telemetry.heartbeat_time > _datalink_last_heartbeat_gcs) {
 						status.data_link_lost = false;
 						_status_changed = true;
 
+						if (!armed.armed) {
+							// make sure to report preflight check failures to a connecting GCS
+							PreFlightCheck::preflightCheck(&mavlink_log_pub, status, status_flags,
+										       _arm_requirements.global_position, true, true, hrt_elapsed_time(&_boot_timestamp));
+						}
+
 						if (_datalink_last_heartbeat_gcs != 0) {
 							mavlink_log_info(&mavlink_log_pub, "Data link regained");
 						}