How to persist boot messages on USB live image?
 

I've a headless custom device that boots from USB using live image. Since I cannot access the device through the network, I need a way to persist boot related log files (dmesg, boot.log, messages) to view them on another computer. I also need to know how can I run custom commands on startup of live image. Thank you.

If you do not have ability to connect monitor and keyboard, I'd say there is nothing to be done.

This device does not have monitor and keyboard connectivity, but I need some feedback about the boot process.

Maybe RS-232 (Serial port)? I can not think of any other way.