boards/nucleo: fix xtimer configuration
boards/nucleo: fix xtimer configuration

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