pc-bios/s390: add s390-netboot.img

pc-bios/s390: add s390-netboot.img

It's already possible to do a network boot of an s390x guest with an
external netboot image based on a Linux installation, but it would
be much more convenient if the s390-ccw firmware supported network
booting right out of the box, without the need to assemble such an
external image first.

This is an s390-netboot.img that can be used for network booting.
You can download a combined kernel + initrd image via TFTP
by starting QEMU for example with:

 qemu-system-s390x ... -device virtio-net,netdev=n1,bootindex=1 \
       -netdev user,id=n1,tftp=/path/to/tftp,bootfile=kernel.img

Note that this version does not support downloading via config
files (i.e. pxelinux config files or .INS config files) yet. This
will be added later.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>