iPXE does not currently provide EFI_PXE_BASE_CODE_PROTOCOL: this
causes failures when chainloading bootloaders such as shim.efi which
assume that this protocol will be present.

Provide the ability to work around these problems via the build
configuration option EFI_DOWNGRADE_UX.  If this option is enabled,
then we will not install our usual EFI_LOAD_FILE_PROTOCOL
implementation, thereby allowing the platform firmware to install its
own EFI_PXE_BASE_CODE_PROTOCOL implementation on top of our
EFI_SIMPLE_NETWORK_PROTOCOL handle.

A somewhat major side-effect of this workaround is that almost all
iPXE features will be disabled.

This configuration option will be removed in future when support for
EFI_PXE_BASE_CODE_PROTOCOL is added.

Requested-by: Laszlo Ersek <lersek@redhat.com>
Requested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>