Devices that support jumbo frames will currently default to the
largest possible MTU.  This assumption is valid for virtual adapters
such as virtio-net, where the MTU must have been configured by a
system administrator, but is unsafe in the general case of a physical
adapter.

Default to the standard Ethernet MTU, unless explicitly overridden
either by the driver or via the ${netX/mtu} setting.

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