AP_SBusOut: add and use AP_SBUSOUTPUT_ENABLED

.... which will allow periphs to instantiate this if they really feel like it, and for it to be removed on smaller boards on the custom build server (and potentially on lower-specced boards.

libraries/AP_SBusOut/AP_SBusOut.cpp

@@ -37,6 +37,11 @@
  * POSSIBILITY OF SUCH DAMAGE.
  *
  */
+
+#include "AP_SBusOut_config.h"
+
+#if AP_SBUSOUTPUT_ENABLED
+
 #include "AP_SBusOut.h"
 #include <AP_Math/AP_Math.h>
 #include <AP_SerialManager/AP_SerialManager.h>
@@ -189,3 +194,4 @@ void AP_SBusOut::init() {
     sbus1_uart = serial_manager->find_serial(AP_SerialManager::SerialProtocol_Sbus1,0);
 }
 
+#endif  // AP_SBUSOUTPUT_ENABLED

libraries/AP_SBusOut/AP_SBusOut.h

@@ -7,6 +7,10 @@
 
 #pragma once
 
+#include "AP_SBusOut_config.h"
+
+#if AP_SBUSOUTPUT_ENABLED
+
 #include <AP_HAL/AP_HAL.h>
 #include <AP_Param/AP_Param.h>
 
@@ -35,3 +39,5 @@ private:
     AP_Int16 sbus_rate;
     bool initialised;
 };
+
+#endif  // AP_SBUSOUTPUT_ENABLED

libraries/AP_SBusOut/AP_SBusOut_config.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <AP_HAL/AP_HAL_Boards.h>
+
+#ifndef AP_SBUSOUTPUT_ENABLED
+#define AP_SBUSOUTPUT_ENABLED 1
+#endif