AP_Relay: allow for no relay pin

libraries/AP_Relay/AP_Relay.cpp

@@ -17,35 +17,50 @@ extern const AP_HAL::HAL& hal;
 #elif CONFIG_HAL_BOARD == HAL_BOARD_APM2 || CONFIG_HAL_BOARD == HAL_BOARD_AVR_SITL
 #define RELAY_PIN 26
 #else
-#error "no RELAY_PIN defined for this board"
+// no relay for this board
+#define RELAY_PIN -1
 #endif
 
 void AP_Relay::init() {
+#if RELAY_PIN != -1
     hal.gpio->pinMode(RELAY_PIN, GPIO_OUTPUT);
+#endif
 }
 
 void AP_Relay::on() {
+#if RELAY_PIN != -1
     hal.gpio->write(RELAY_PIN, 1);
+#endif
 }
 
 
 void AP_Relay::off() {
+#if RELAY_PIN != -1
     hal.gpio->write(RELAY_PIN, 0);
+#endif
 }
 
 
 void AP_Relay::toggle() {
+#if RELAY_PIN != -1
     bool ison = hal.gpio->read(RELAY_PIN);
     if (ison)
         off();
     else
         on();
+#endif
 }
 
 void AP_Relay::set(bool status){
+#if RELAY_PIN != -1
     hal.gpio->write(RELAY_PIN, status);
+#endif
 }
 
 bool AP_Relay::get() {
+#if RELAY_PIN != -1
     return hal.gpio->read(RELAY_PIN); 
+#else
+    return false;
+#endif
 }