Plane: support up to 16 output channels

the extra channels can be used via SBUS output

ArduPlane/Parameters.cpp

@@ -1170,6 +1170,14 @@ const AP_Param::Info Plane::var_info[] = {
     // @Path: ../libraries/RC_Channel/RC_Channel.cpp,../libraries/RC_Channel/RC_Channel_aux.cpp
     GGROUP(rc_14,                    "RC14_", RC_Channel_aux),
 
+    // @Group: RC15_
+    // @Path: ../libraries/RC_Channel/RC_Channel.cpp,../libraries/RC_Channel/RC_Channel_aux.cpp
+    GGROUP(rc_15,                    "RC15_", RC_Channel_aux),
+
+    // @Group: RC16_
+    // @Path: ../libraries/RC_Channel/RC_Channel.cpp,../libraries/RC_Channel/RC_Channel_aux.cpp
+    GGROUP(rc_16,                    "RC16_", RC_Channel_aux),
+    
     // @Group: RLL2SRV_
     // @Path: ../libraries/APM_Control/AP_RollController.cpp
 	GOBJECT(rollController,         "RLL2SRV_",   AP_RollController),

ArduPlane/Parameters.h

@@ -284,6 +284,8 @@ public:
         k_param_quadplane,
         k_param_rtl_radius,
         k_param_land_then_servos_neutral,
+        k_param_rc_15,
+        k_param_rc_16,
 
         //
         // 210: flight modes
@@ -527,6 +529,8 @@ public:
     RC_Channel_aux rc_12;
     RC_Channel_aux rc_13;
     RC_Channel_aux rc_14;
+    RC_Channel_aux rc_15;
+    RC_Channel_aux rc_16;
     uint8_t _dummy;
 
     Parameters() :
@@ -546,6 +550,8 @@ public:
         rc_12                                   (CH_12),
         rc_13                                   (CH_13),
         rc_14                                   (CH_14),
+        rc_15                                   (CH_15),
+        rc_16                                   (CH_16),
         _dummy(0)
         {}
 };