AP_Landing: Implement is_on_approach for deepstall

(Fixes a disarm in flight)

libraries/AP_Landing/AP_Landing.cpp

@@ -308,6 +308,7 @@ bool AP_Landing::is_on_approach(void) const
     case TYPE_STANDARD_GLIDE_SLOPE:
         return type_slope_is_on_approach();
     case TYPE_DEEPSTALL:
+        return deepstall.is_on_approach();
     default:
         return false;
     }

libraries/AP_Landing/AP_Landing_Deepstall.cpp

@@ -367,6 +367,11 @@ bool AP_Landing_Deepstall::is_flying_forward(void) const
     return stage != DEEPSTALL_STAGE_LAND;
 }
 
+bool AP_Landing_Deepstall::is_on_approach(void) const
+{
+    return stage == DEEPSTALL_STAGE_LAND;
+}
+
 bool AP_Landing_Deepstall::get_target_altitude_location(Location &location)
 {
     memcpy(&location, &landing_point, sizeof(Location));

libraries/AP_Landing/AP_Landing_Deepstall.h

@@ -102,6 +102,7 @@ private:
     int32_t get_target_airspeed_cm(void) const;
     bool is_throttle_suppressed(void) const;
     bool is_flying_forward(void) const;
+    bool is_on_approach(void) const;
     bool terminate(void);
     void log(void) const;