core/include/kernel_defines.h: add index_of() macro
core/include/kernel_defines.h: add index_of() macro

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