My solution was to download all kirkwood stuff (e.g. from the ALARM home page) and extract it to an usb stick formatted with etx2. Before booting the boot parameters need some adjustment
setenv bootargs_root ubi.mtd=1 root=/dev/sda1 rootdelay=10
which makes the Sheeva boot from the usb stick. And after "mount /dev/ubi0.. /mnt" the world keeps revolving.