AP_AHRS: move is_ext_nav_used_for_yaw to parent

libraries/AP_AHRS/AP_AHRS.h

@@ -183,7 +183,10 @@ public:
 
     // see if EKF lane switching is possible to avoid EKF failsafe
     virtual void check_lane_switch(void) {}
-    
+
+    // check whether external navigation is providing yaw.  Allows compass pre-arm checks to be bypassed
+    virtual bool is_ext_nav_used_for_yaw(void) const { return false; }
+
     // Euler angles (radians)
     float roll;
     float pitch;

libraries/AP_AHRS/AP_AHRS_NavEKF.h

@@ -280,8 +280,8 @@ public:
 
     void Log_Write();
 
-    // check whether compass can be bypassed for arming check in case when external navigation data is available 
-    bool is_ext_nav_used_for_yaw(void) const;
+    // check whether external navigation is providing yaw.  Allows compass pre-arm checks to be bypassed
+    bool is_ext_nav_used_for_yaw(void) const override;
 
     // these are only out here so vehicles can reference them for parameters
 #if HAL_NAVEKF2_AVAILABLE