[efi] Add our own EFI_LOAD_FILE_PROTOCOL implementation

[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>