pseries: Update SLOF firmware image to qemu-slof-20171214

The main changes are:
- able to handle more devices with specified bootindex;
- implements flatten device tree rendering, for both QEMU and guest kernel.

The full list is:
  > boot: use a temporary bootdev-buf
  > boot: do not concatenate bootdev
  > libvirtio: Mark struct virtio_scsi_req_cmd as packed
  > fdt: Implement "fdt-fetch" method for client interface
  > rtas: Store RTAS address and entry in the device tree
  > board-qemu: Fix slof-build-id length
  > fdt: Pass the resulting device tree to QEMU
  > fdt: Fix version and add a word for FDT header size
  > tree: Rework set-chosen-cpu and store /chosen ihandle and phandle
  > node: Add some documentation
  > Revert various SLOF-to-QEMU private hypercalls
  > Use input-device and output-device
  > netboot: Create bootp-response when bootp is used
  > libnet/ipv6: assign times_asked value directly
  > usb-xhci: Reset ERSTSZ together with ERSTBA
  > virtio-net: rework the driver to support multiple open
  > board-qemu: add private hcall to inform host on "phandle" update

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>