Add PXE-EFI64 and .ISO recovery images

Reviewed By: TBD

In order to install ONIE on a blank x86 machine (or recovery a system
with a broken ONIE) we need a recovery mechanism and recovery image.

This patch introduces two recovery mechanisms:

- PXE boot an ONIE recovery image via the network

- locally boot an ONIE recovery ISO image from a USB stick

To enable building the PXE-EFI64 and ISO recovery images add this line
to the platform's machine.make file:

  PXE_EFI64_ENABLE = yes

The build of the images flows as follows:

- Make a recovery initrd based on the ONIE sysroot that also includes
  the ONIE updater image.

- Make a hybrid .iso image containing the ONIE kernel and recovery
  intrd.  You can "dd" this .iso to a USB memstick and use it for
  recovery.

- Convert the .iso to a PXE-EFI64 bootable image using GRUB.  You can
  PXE boot this image on UEFI64 machines and use it for recovery.

Once booted, the system is in "ONIE Recovery" mode without full
functionality.  In particular you cannot install NOS images while in
ONIE recovery.  You can, however, update/install ONIE itself.

The recovery image contains a full ONIE updater image you can use to
recovery the system.  Type this:

  ONIE-RECOVERY:/ # onie-self-update file:///lib/onie/onie-updater

The PXE-EFI64 image also contains a GRUB menu entry for jumping
straight to installing the ONIE updater image.

Testing Done:

- Booted a UEFI64 system via PXE and installed ONIE

- Booted a UEFI64 system via a USB stick and installed ONIE