Plane: support building quadplane with forced motors class

ArduPlane/quadplane.cpp

@@ -303,7 +303,7 @@ bool QuadPlane::setup(void)
         goto failed;
     }
 
-#ifdef AP_MOTORS_CLASS
+#ifdef AP_MOTORS_FORCE_CLASS
     RC_Channel_aux::set_aux_channel_default(RC_Channel_aux::k_motor1, CH_5);
     RC_Channel_aux::set_aux_channel_default(RC_Channel_aux::k_motor2, CH_6);
     RC_Channel_aux::set_aux_channel_default(RC_Channel_aux::k_motor4, CH_8);

ArduPlane/quadplane.h

@@ -8,7 +8,14 @@
 #include <AC_WPNav/AC_WPNav.h>
 
 // uncomment this to force a different motor class
-// #define AP_MOTORS_CLASS AP_MotorsTri
+// #define AP_MOTORS_FORCE_CLASS AP_MotorsTri
+
+
+#ifdef AP_MOTORS_FORCE_CLASS
+#define AP_MOTORS_CLASS AP_MOTORS_FORCE_CLASS
+#else
+#define AP_MOTORS_CLASS AP_MotorsMulticopter
+#endif
 
 /*
   QuadPlane specific functionality