Add support for boards with different name of serial USB connection (#6)

* Replace SerialUSB by SERIAL_PORT_USBVIRTUAL

Support boards where USB serial is named differently. E.g. the
Adafruit Feather M0 uses Serial1 as name for serial USB port.

Author: stlehmann

src/samd/ArduinoLowPower.cpp

@@ -17,7 +17,7 @@ void ArduinoLowPowerClass::idle(uint32_t millis) {
 
 void ArduinoLowPowerClass::sleep() {
 	bool restoreUSBDevice = false;
-	if (SerialUSB) {
+	if (SERIAL_PORT_USBVIRTUAL) {
 		USBDevice.standby();
 	} else {
 		USBDevice.detach();
@@ -100,4 +100,4 @@ void ArduinoLowPowerClass::attachInterruptWakeup(uint32_t pin, voidFuncPtr callb
 
 ArduinoLowPowerClass LowPower;
 
-#endif // ARDUINO_ARCH_SAMD
+#endif // ARDUINO_ARCH_SAMD