boards/samr21-xpro: add a note about EDBG UART limitation
boards/samr21-xpro: add a note about EDBG UART limitation

ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT