boards/common/silabs: make series defines explicit
boards/common/silabs: make series defines explicit

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