pseries: Update SLOF firmware image

Various fixes in this update, the full list is:

  > qemu-bootlist: Take the "-boot strict=off" setting properly into account
  > virtio-scsi: initialize vring avail queue buffers
  > virtio: Remove global variables in block and 9p driver
  > Remove superfluous checkpoints in tree.fs
  > Provide "write" function in the disk-label package
  > virtio: Implement block write support
  > scsi: Add SCSI block write support
  > deblocker: Add a 'write' function
  > virtio-scsi: Fix descriptor order for SCSI WRITE commands
  > board-qemu: Add a possibility to use hvterm input instead of USB keyboard
  > Do not try to use virtio-gpu in VGA mode
  > virtio: Fix stack comment of virtio-blk-read
  > envvar: Do not read default values for /options from the NVRAM anymore
  > envvar: Set properties in /options during "(set-defaults)"

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