boards/atmega256rfr2: Add Kconfig symbols
boards/atmega256rfr2: Add Kconfig symbols

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