makefiles/suit: don't allow non-owner to read private key
makefiles/suit: don't allow non-owner to read private key

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