[efi] Update to latest EDK2 headers

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[autoboot] Split main control flow out of main() into a new function ipxe()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Add last_opened_snpdev()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Expose downloaded images via EFI_SIMPLE_FILE_SYSTEM_PROTOCOL

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>
[efi] Add our own EFI_LOAD_FILE_PROTOCOL implementation

When iPXE is used as a UEFI driver, the UEFI PXE base code currently
provides the TCP/IP stack, network protocols, and user interface.
This represents a substantial downgrade from the standard BIOS iPXE
user experience.

Fix by installing our own EFI_LOAD_FILE_PROTOCOL implementation which
initiates the standard iPXE boot procedure.  This upgrades the UEFI
iPXE user experience to match the standard BIOS iPXE user experience.

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