Xbee 802.15.4 modem driver for Arduino

This radio driver allows to use an XBee 802.15.4 modem with an Arduino board, using the official Arduino Wireless/SD shield (see http://arduino.cc/en/Main/ArduinoWirelessShield) for connection.

I think you can throw out this variable and use `XBEE_UART_LINK` instead throughout the driver.