boards/atmega256rfr2-xpro add USEC_ADJUST values
boards/atmega256rfr2-xpro add USEC_ADJUST values

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