cortexm_common: Determine next thread before unscheduling
cortexm_common: Determine next thread before unscheduling

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