dist/tools/kconfiglib: avoid rewriting /dev/null
dist/tools/kconfiglib: avoid rewriting /dev/null

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