AP_GPS: Always send GPS2_RAW after we've seen the second GPS

This significantly improves the GCS's ability to correctly display whats
happening with the second GPS.

libraries/AP_GPS/AP_GPS.cpp

@@ -602,6 +602,9 @@ found_gps:
         timing[instance].last_message_time_ms = now;
         timing[instance].delta_time_ms = GPS_TIMEOUT_MS;
         new_gps->broadcast_gps_type();
+        if (instance == 1) {
+            has_had_second_instance = true;
+        }
     }
 }
 
@@ -989,7 +992,8 @@ void AP_GPS::send_mavlink_gps_raw(mavlink_channel_t chan)
 #if GPS_MAX_RECEIVERS > 1
 void AP_GPS::send_mavlink_gps2_raw(mavlink_channel_t chan)
 {
-    if (num_instances < 2 || status(1) <= AP_GPS::NO_GPS) {
+    // always send the message once we've ever seen the GPS
+    if (!has_had_second_instance) {
         return;
     }
 

libraries/AP_GPS/AP_GPS.h

@@ -579,6 +579,9 @@ private:
 
     // used for flight testing with GPS loss
     bool _force_disable_gps;
+
+    // used to ensure we continue sending status messages if we ever detected the second GPS
+    bool has_had_second_instance;
 };
 
 namespace AP {