s390x/ipl: Load network boot image

Load the network boot image into guest RAM when the boot
device selected is a network device. Use some of the reserved
space in IplBlockCcw to store the start address of the netboot
image.

A user could also use 'chreipl'(diag 308/5) to change the boot device.
So every time we update the IPLB, we need to verify if the selected
boot device is a network device so we can appropriately load the
network boot image.

Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
