AP_Arming: check for only first compass being disabled

If only the first compass is disabled, users may expect other enabled compasses to be used but they won't be

libraries/AP_Arming/AP_Arming.cpp

@@ -414,6 +414,12 @@ bool AP_Arming::compass_checks(bool report)
     if ((checks_to_perform) & ARMING_CHECK_ALL ||
         (checks_to_perform) & ARMING_CHECK_COMPASS) {
 
+        // check for first compass being disabled but 2nd or 3rd being enabled
+        if (!_compass.use_for_yaw(0) && (_compass.get_num_enabled() > 0)) {
+            check_failed(ARMING_CHECK_COMPASS, report, "Compass1 disabled but others enabled");
+            return false;
+        }
+
         // avoid Compass::use_for_yaw(void) as it implicitly calls healthy() which can
         // incorrectly skip the remaining checks, pass the primary instance directly
         if (!_compass.use_for_yaw(0)) {