pc-bios/s390-ccw: Link libnet into the netboot image and do the TFTP load

Most of the code has been taken from SLOF's netload.c file. Now we
can finally load an image via TFTP and execute the downloaded kernel.

Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1499863793-18627-12-git-send-email-thuth@redhat.com>
Tested-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>