AP_AHRS: allow override of default EKF type

libraries/AP_AHRS/AP_AHRS.cpp

@@ -24,6 +24,10 @@
 
 extern const AP_HAL::HAL& hal;
 
+#ifndef HAL_AHRS_EKF_TYPE_DEFAULT
+#define HAL_AHRS_EKF_TYPE_DEFAULT 3
+#endif
+
 // table of user settable parameters
 const AP_Param::GroupInfo AP_AHRS::var_info[] = {
     // index 0 and 1 are for old parameters that are no longer not used
@@ -127,9 +131,9 @@ const AP_Param::GroupInfo AP_AHRS::var_info[] = {
     // @Param: EKF_TYPE
     // @DisplayName: Use NavEKF Kalman filter for attitude and position estimation
     // @Description: This controls which NavEKF Kalman filter version is used for attitude and position estimation
-    // @Values: 0:Disabled,2:Enable EKF2,3:Enable EKF3
+    // @Values: 0:Disabled,2:Enable EKF2,3:Enable EKF3,11:ExternalAHRS
     // @User: Advanced
-    AP_GROUPINFO("EKF_TYPE",  14, AP_AHRS, _ekf_type, 3),
+    AP_GROUPINFO("EKF_TYPE",  14, AP_AHRS, _ekf_type, HAL_AHRS_EKF_TYPE_DEFAULT),
 #endif
 
     // @Param: CUSTOM_ROLL