AP_ExternalAHRS: added support for GPS disable and fwd flight

allow backends to determine if we are in fixed wing flight and/or the
GPS is disabled by the user

libraries/AP_ExternalAHRS/AP_ExternalAHRS.h

@@ -164,6 +164,11 @@ public:
         float temperature; // degC
     } airspeed_data_message_t;
 
+    // set GNSS disable for auxillary function GPS_DISABLE
+    void set_gnss_disable(bool disable) {
+        gnss_is_disabled = disable;
+    }
+
 protected:
 
     enum class OPTIONS {
@@ -193,6 +198,9 @@ private:
     }
 
     uint32_t last_log_ms;
+
+    // true when user has disabled the GNSS
+    bool gnss_is_disabled;
 };
 
 namespace AP {

libraries/AP_ExternalAHRS/AP_ExternalAHRS_backend.cpp

@@ -17,6 +17,7 @@
  */
 
 #include "AP_ExternalAHRS_backend.h"
+#include <AP_AHRS/AP_AHRS.h>
 
 #if HAL_EXTERNAL_AHRS_ENABLED
 
@@ -37,5 +38,10 @@ bool AP_ExternalAHRS_backend::option_is_set(AP_ExternalAHRS::OPTIONS option) con
     return frontend.option_is_set(option);
 }
 
+bool AP_ExternalAHRS_backend::in_fly_forward(void) const
+{
+    return AP::ahrs().get_fly_forward();
+}
+
 #endif  // HAL_EXTERNAL_AHRS_ENABLED
 

libraries/AP_ExternalAHRS/AP_ExternalAHRS_backend.h

@@ -58,6 +58,18 @@ protected:
         frontend.set_default_sensors(sensors);
     }
 
+    /*
+      return true if the GNSS is disabled
+     */
+    bool gnss_is_disabled(void) const {
+        return frontend.gnss_is_disabled;
+    }
+
+    /*
+      return true when we are in fixed wing flight
+     */
+    bool in_fly_forward(void) const;
+
 private:
     AP_ExternalAHRS &frontend;
 };