boards/common/iotlab: add CONFIG_ZTIMER_USEC
boards/common/iotlab: add CONFIG_ZTIMER_USEC

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