[efi] Veto the Dhcp6Dxe driver on all platforms

[efi] Veto the Dhcp6Dxe driver on all platforms

The reference implementation of Dhcp6Dxe in EDK2 has a fatal flaw: the
code in EfiDhcp6Stop() will poll the network in a tight loop until
either a response is received or a timer tick (at TPL_CALLBACK)
occurs.  When EfiDhcp6Stop() is called at TPL_CALLBACK or higher, this
will result in an endless loop and an apparently frozen system.

Since this is the reference implementation of Dhcp6Dxe, it is likely
that almost all platforms have the same problem.

Fix by vetoing the broken driver.  If the upstream driver is ever
fixed and a new version number issued, then we could plausibly test
against the version number exposed via the driver binding protocol.

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