Just tried and I got something new:
cat /dev/ttyACM0
0xFF,0xFF,0xFF
0x05-ERRS
0x1F,0x84,0x01
A second and third press of the button, fixed it. I haven't seen that before.
I currently have a valid image in flash.
Things are currently working for me, but I am doing the flash writing from my SoC not the firmware.