Plane: add a default Navigation option as explained in the param docs

ArduPlane/navigation.cpp

@@ -6,6 +6,11 @@
 void Plane::set_nav_controller(void)
 {
     switch ((AP_Navigation::ControllerType)g.nav_controller.get()) {
+
+    default:
+    case AP_Navigation::CONTROLLER_DEFAULT:
+        // fall through to L1 as default controller
+
     case AP_Navigation::CONTROLLER_L1:
         nav_controller = &L1_controller;
         break;

libraries/AP_Navigation/AP_Navigation.h

@@ -106,7 +106,8 @@ public:
 	// select which navigation controller to use by setting the
 	// NAV_CONTROLLER parameter
 	enum ControllerType {
-		CONTROLLER_L1     = 1
+	    CONTROLLER_DEFAULT      = 0,
+		CONTROLLER_L1           = 1
 	};
 };