cpu: boards: stm32f2/f4/f7: rework clock configuration and init
cpu: boards: stm32f2/f4/f7: rework clock configuration and init

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