[efi] Check for presence of UNDI in NII protocol

iPXE itself exposes a dummy NII protocol with no UNDI.  Avoid
potentially dereferencing a NULL pointer by checking for a non-zero
UNDI address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Include NII driver within "snp" and "snponly" build targets

End users almost certainly don't care whether the underlying interface
is SNP or NII/UNDI.  Try to minimise surprise and unnecessary
documentation by including the NII driver whenever the SNP driver is
requested.

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