[efi] Add support for driving EFI_MANAGED_NETWORK_PROTOCOL devices

[efi] Add support for driving EFI_MANAGED_NETWORK_PROTOCOL devices

We want exclusive access to the network device, both for performance
reasons and because we perform operations such as EAPoL that affect
the entire link.  We currently drive the network card via either a
native hardware driver or via the SNP or NII/UNDI interfaces, both of
which grant us this exclusive access.

Add an alternative driver that drives the network card non-exclusively
via the EFI_MANAGED_NETWORK_PROTOCOL interface.  This can function as
a fallback for situations where neither SNP nor NII/UNDI interfaces
are functional, and also opens up the possibility of non-destructively
installing a temporary network device over which to download the
autoexec.ipxe script.

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