AP_AHRS: Set the new gps_fusion notify flag

Uses EKF filter state to set the new gps_fusion notify flag. This allows
the GCS and notify devices to specifically be notified if the GPS is or
is not usable.

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -192,6 +192,9 @@ void AP_AHRS_NavEKF::update_EKF2(void)
                 }
             }
             _accel_ef_ekf_blended = _accel_ef_ekf[primary_imu>=0?primary_imu:_ins.get_primary_accel()];
+            nav_filter_status filt_state;
+            EKF2.getFilterStatus(-1,filt_state);
+            AP_Notify::flags.gps_fusion = filt_state.flags.using_gps; // Drives AP_Notify flag for usable GPS.
         }
     }
 }
@@ -260,6 +263,9 @@ void AP_AHRS_NavEKF::update_EKF3(void)
                 }
             }
             _accel_ef_ekf_blended = _accel_ef_ekf[_ins.get_primary_accel()];
+            nav_filter_status filt_state;
+            EKF3.getFilterStatus(-1,filt_state);
+            AP_Notify::flags.gps_fusion = filt_state.flags.using_gps; // Drives AP_Notify flag for usable GPS.
         }
     }
 }