cpu/atmega2560: Add Kconfig symbols
cpu/atmega2560: Add Kconfig symbols

miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,OlegHahm/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,ant9000/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OTAkeys/RIOT