Plane: support better build time selection of multicopter frame type

makes it possible to do "make sitl-tri"

ArduPlane/quadplane.cpp

@@ -217,12 +217,14 @@ const AP_Param::GroupInfo QuadPlane::var_info[] = {
     // @Increment: 1
     AP_GROUPINFO("TRAN_PIT_MAX", 29, QuadPlane, transition_pitch_max, 3),
 
+#if FRAME_CONFIG == MULTICOPTER_FRAME
     // @Param: FRAME_CLASS
     // @DisplayName: Frame Class
     // @Description: Controls major frame class for multicopter component
     // @Values: 0:Quad, 1:Hexa, 2:Octa, 3:OctaQuad, 4:Y6
     // @User: Standard
     AP_GROUPINFO("FRAME_CLASS", 30, QuadPlane, frame_class, 0),
+#endif
 
     // @Param: FRAME_TYPE
     // @DisplayName: Frame Type (+, X or V)
@@ -318,7 +320,7 @@ bool QuadPlane::setup(void)
         goto failed;
     }
 
-#ifdef AP_MOTORS_FORCE_CLASS
+#if FRAME_CONFIG == TRI_FRAME
     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

@@ -7,12 +7,20 @@
 #include <AC_AttitudeControl/AC_PosControl.h>
 #include <AC_WPNav/AC_WPNav.h>
 
-// uncomment this to force a different motor class
-// #define AP_MOTORS_FORCE_CLASS AP_MotorsTri
+/*
+  frame types for quadplane build. Most case be set with
+  parameters. Those that can't are listed here and chosen with a build
+  time FRAME_CONFIG parameter
+ */
+#define MULTICOPTER_FRAME 1
+#define TRI_FRAME 2
 
+#ifndef FRAME_CONFIG
+# define FRAME_CONFIG MULTICOPTER_FRAME
+#endif
 
-#ifdef AP_MOTORS_FORCE_CLASS
-#define AP_MOTORS_CLASS AP_MOTORS_FORCE_CLASS
+#if FRAME_CONFIG == TRI_FRAME
+#define AP_MOTORS_CLASS AP_MotorsTri
 #else
 #define AP_MOTORS_CLASS AP_MotorsMulticopter
 #endif
@@ -110,7 +118,9 @@ private:
     AC_PID                  pid_accel_z{0.3, 1, 0, 800, 10, 0.02};
     AC_PI_2D                pi_vel_xy{0.7, 0.35, 1000, 5, 0.02};
 
+#if FRAME_CONFIG == MULTICOPTER_FRAME
     AP_Int8 frame_class;
+#endif
     AP_Int8 frame_type;
     
     AP_MOTORS_CLASS *motors;