[efi] Add NII / UNDI driver

Some UEFI network drivers provide a software UNDI interface which is
exposed via the Network Interface Identifier Protocol (NII), rather
than providing a Simple Network Protocol (SNP).

The UEFI platform firmware will usually include the SnpDxe driver,
which attaches to NII and provides an SNP interface.  The SNP
interface is usually provided on the same handle as the underlying NII
device.  This causes problems for our EFI driver model: when
efi_driver_connect() detaches existing drivers from the handle it will
cause the SNP interface to be uninstalled, and so our SNP driver will
not be able to attach to the handle.  The platform firmware will
eventually reattach the SnpDxe driver and may attach us to the SNP
handle, but we have no way to prevent other drivers from attaching
first.

Fix by providing a driver which can attach directly to the NII
protocol, using the software UNDI interface to drive the network
device.

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