[efi] Add iPXE download protocol

[efi] Add iPXE download protocol

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>