Wired and Wireless with Etables

[ray <ray@aarden.us>]

I would like to determine how to configure dom0 to use either wireless or wired connections automatically depending upon which are available.

I am trying to get Xen4.9 up and Debian 9 on a Toshiba laptop with only a wireless and a wired (enx0) connection through a docking station.  I am trying to use:  https://wiki.debian.org/BridgeNetworkConnections#Bridging_with_a_wireless_NIC
This laptop is a personal workstation implementing a variety of vms and OSs. Testing wireless internet connection is using JetPack 4G AP and I intend to use various public/private wireless APs.  

It seems the way to do this is to bond wlan0 and eth0 as bond0, then build a bridge for domus with bond0.  Etables requires the bridge MAC to be defined in the rules.  I do not know how to assign a MAC to the bridge since I don't know which interface came up.

How can I set up bridging without knowing which interface is up?

Thanks,
Ray