AP_Arming: Allow arming when forcing DCM and no GPS configured

* If we force AP to use DCM, and disable all GPS's, we never expect a
  home location
* Allow users to arm DCM with all pre-arm checks enabled in this special
  case
* This allows using AP as a simple stabilizer (FBWA) on plane
* If a user uses EK3 and does not have any GPS's configured (ie, no
  GPS), they can now arm

Signed-off-by: Ryan Friedman <25047695+Ryanf55@users.noreply.github.com>

libraries/AP_Arming/AP_Arming.cpp

@@ -636,7 +636,13 @@ bool AP_Arming::gps_checks(bool report)
             }
         }
 
-        if (!AP::ahrs().home_is_set()) {
+        // Only check we have an AHRS home when we have at least 1 GPS enabled.
+        // This allows arming in DCM without GPS if the user doesn't have a GPS installed.
+        bool expects_home_location = gps.num_sensors() > 0;
+#if AP_AHRS_DCM_ENABLED
+        expects_home_location = (AP_AHRS::EKFType)AP::ahrs().get_ekf_type() != AP_AHRS::EKFType::DCM;
+#endif
+        if (expects_home_location && !AP::ahrs().home_is_set()) {
             check_failed(ARMING_CHECK_GPS, report, "AHRS: waiting for home");
             return false;
         }