[ecm] Use ACPI-provided system-specific MAC address if present

Use the "system MAC address" provided within the DSDT/SSDT if such an
address is available and has not already been assigned to a network
device.

Tested-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>