boards/nrf52840-mdk: Add Kconfig symbols
boards/nrf52840-mdk: Add Kconfig symbols

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