Exploit the redefinition of iPXE error codes to include a "platform
error code" to allow for meaningful conversion of EFI_STATUS values to
iPXE errors and vice versa.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Expose iPXE's images as a UEFI file system, allowing the booted image
to access all images downloaded by iPXE.

This functionality is complementary to the custom iPXE download
protocol.  The iPXE download protocol allows a booted image to utilise
iPXE to download arbitrary URIs, but requires the booted image to
specifically support the custom iPXE download protocol.  The new
functionality limits the booted image to accessing only files that
were already downloaded by iPXE (e.g. as part of a script), but can
work with any generic UEFI image (e.g. the UEFI shell).  Both
protocols are provided simultaneously, and are attached to the SNP
device handle.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

iPXE exposes some extended capabilities via the PXE FILE API to allow
NBPs such as pxelinux to use protocols other than TFTP.  Provide an
equivalent interface as a UEFI protocol so that EFI binaries may also
take advantage of iPXE's extended capabilities.

This can be used with a patched version of elilo, for example:

  http://comments.gmane.org/gmane.comp.boot-loaders.elilo.general/147

Signed-off-by: Michael Brown <mcb30@ipxe.org>