OK thanks for all information. I have ran kvim2-update successfully so I think I am fine.
One day I will also try kvim2-full to install custom Linux like gentoo.
Holding the remote power button brought up menu and I could boot in ubuntu MATE
Life can be simple !
My USB mouse works, but, for some reason, my (Apple) USB keyboard does not work. So I cannot log in to the GUI. I can log in to console and I also have installed ssh service.
update: I can use onscreen keyboard …
But now: how do I boot back into Android ? Perhaps from Linux command line or perhaps from u-boot command line?
Thanks for this excellent 3-in-1 system!