[ecm] Treat ACPI MAC address as being a non-permanent MAC address

[ecm] Treat ACPI MAC address as being a non-permanent MAC address

When applying an ACPI-provided system-specific MAC address, apply it
to netdev->ll_addr rather than netdev->hw_addr.  This allows iPXE
scripts to access the permanent MAC address via the ${netX/hwaddr}
setting (and thereby provides scripts with a mechanism to ascertain
that the NIC is using a MAC address other than its own permanent
hardware address).

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