cpu/stm32/vendor: use submake to fetch CMSIS headers
cpu/stm32/vendor: use submake to fetch CMSIS headers

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