ztimer: add ztimer_spin()
ztimer: add ztimer_spin()

This adds a simple busy waiting function intended for blocking, but very
precise short delays.

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