core: add core_lib to Kconfig
core: add core_lib to Kconfig

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