boards/feather-m0: use UART_UNDEF to enable Serial over STDIO

boards/feather-m0/include/arduino_board.h

@@ -31,6 +31,13 @@ extern "C" {
  */
 #define ARDUINO_LED         (13)
 
+/**
+ * @brief   On-board serial port mapping, stdio is used for Serial
+ */
+#ifndef ARDUINO_UART_DEV
+#define ARDUINO_UART_DEV    UART_UNDEF
+#endif
+
 /**
  * @brief   Look-up table for the Arduino's digital pins
  */