AP_Relay: add option to disable relay and servorelay libraries

libraries/AP_Relay/AP_Relay.cpp

@@ -7,6 +7,9 @@
 
 #include <AP_HAL/AP_HAL.h>
 #include "AP_Relay.h"
+
+#if AP_RELAY_ENABLED
+
 #include <AP_Logger/AP_Logger.h>
 
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
@@ -195,3 +198,5 @@ AP_Relay *relay()
 }
 
 }
+
+#endif  // AP_RELAY_ENABLED

libraries/AP_Relay/AP_Relay.h

@@ -9,6 +9,10 @@
 /// @brief	APM relay control class
 #pragma once
 
+#include "AP_Relay_config.h"
+
+#if AP_RELAY_ENABLED
+
 #include <AP_Param/AP_Param.h>
 
 #define AP_RELAY_NUM_RELAYS 6
@@ -64,3 +68,5 @@ private:
 namespace AP {
     AP_Relay *relay();
 };
+
+#endif  // AP_RELAY_ENABLED

libraries/AP_Relay/AP_Relay_config.h

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