boards/common/stm32l1: introduce common clock config header
boards/common/stm32l1: introduce common clock config header

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