Plane: add QuadPlane singleton

ArduPlane/quadplane.cpp

@@ -597,6 +597,11 @@ QuadPlane::QuadPlane(AP_AHRS_NavEKF &_ahrs) :
 {
     AP_Param::setup_object_defaults(this, var_info);
     AP_Param::setup_object_defaults(this, var_info2);
+
+    if (_singleton != nullptr) {
+        AP_HAL::panic("Can only be one Quadplane");
+    }
+    _singleton = this;
 }
 
 
@@ -3573,3 +3578,5 @@ bool QuadPlane::show_vtol_view() const
 
     return show_vtol;
 }
+
+QuadPlane *QuadPlane::_singleton = nullptr;

ArduPlane/quadplane.h

@@ -43,6 +43,10 @@ public:
     
     QuadPlane(AP_AHRS_NavEKF &_ahrs);
 
+    static QuadPlane *get_singleton() {
+        return _singleton;
+    }
+
     // var_info for holding Parameter information
     static const struct AP_Param::GroupInfo var_info[];
     static const struct AP_Param::GroupInfo var_info2[];
@@ -652,4 +656,6 @@ public:
                                         uint8_t motor_count);
 private:
     void motor_test_stop();
+
+    static QuadPlane *_singleton;
 };