[efi] Drag in MNP driver whenever SNP driver is present

[efi] Drag in MNP driver whenever SNP driver is present

The chainloaded-device-only "snponly" driver already drags in support
for driving SNP, NII, and MNP devices, on the basis that the user
generally doesn't care which UEFI API is used and just wants to boot
from the same network device that was used to load iPXE.

The multi-device "snp" driver already drags in support for driving SNP
and NII devices, but does not drag in support for MNP devices.

There is essentially zero code size overhead to dragging in support
for MNP devices, since this support is always present in any iPXE
application build anyway (as part of the code to download
"autoexec.ipxe" prior to installing our own drivers).

Minimise surprise by dragging in support for MNP devices whenever
using the "snp" driver, following the same reasoning used for the
"snponly" driver.

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