RC_Channel: add AUTOTUNE to init_aux_function routine

Also, cark it in SITL if we fail to initialise an aux function

ArduCopter/RC_Channel.cpp

@@ -93,6 +93,7 @@ void RC_Channel_Copter::init_aux_function(const aux_func_t ch_option, const aux_
     case SAVE_WP:
     case RESETTOARMEDYAW:
     case AUTO:
+    case AUTOTUNE:
         break;
     default:
         RC_Channel::init_aux_function(ch_option, ch_flag);

libraries/RC_Channel/RC_Channel.cpp

@@ -432,6 +432,9 @@ void RC_Channel::init_aux_function(const aux_func_t ch_option, const aux_switch_
         break;
     default:
         gcs().send_text(MAV_SEVERITY_WARNING, "Failed to initialise RC function (%u)", ch_option);
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        AP_HAL::panic("Initialisation failed");
+#endif
         break;
     }
 }