So, the arduino yunserialterminal.ino program that talks to the Atheros /dev/ttyAMA0 expects default 250,000 baud, I installed stty to check what its settings are as default and its 9600 baud. So if you change the arduino yunserialterminal.ino program to use 9600 baud can get comms between arduino and Atheros and use the Atheros serial linux terminal via usb port on Yun.
Also installed yunbridge but that fails trying to open /dev/ttyS0, seems this is because of a change to setup at a certain revision of openwrt that no longer sets up ttyS0
Actually I probably do not need yunbridge at all, can run a python script on the linux side that processes the serial data from ttyATH0 how I want.
However I may want to access console over the arduino usb cable if I lock my self out learning how to configure the firewall and forwarding, so I need a way to start linux console from the python program that sits and monitors serial port from arduino and looks for a command string to fire up the console.
Can stop the console OK and set up the port for comms to arduino using instructions from this article
https://openwrt.org/docs/guide-user/hardware/terminate.console.on.serial
Then can set terminal speed to 115200 using
stty -F /dev/ttyATH0 115200
so :-
Is there any way to fire up the console on ttyATH0 again, ideally at the faster speed of 115200