AP_HAL_Empty: SPIDevice: implement fullduplex transfer

libraries/AP_HAL_Empty/SPIDevice.h

@@ -49,6 +49,13 @@ public:
         return true;
     }
 
+    /* See AP_HAL::SPIDevice::transfer_fullduplex() */
+    bool transfer_fullduplex(const uint8_t *send, uint8_t *recv,
+                             uint32_t len) override
+    {
+        return true;
+    }
+
     /* See AP_HAL::Device::get_semaphore() */
     AP_HAL::Semaphore *get_semaphore()
     {