Ok...back to the drawing board
The Hub arrived and is working well.
It took me some time to get the retrogame app build and settings modified.
I tried mounting the USB in the built in file manager, copying the files, creating folders and modified rc.local.
That all would have been trivial if I did't use the Gameboy screen and a wireless presenter keyboard with german layout and no special Characters...
At the end I gave up at the last step, how important can a "&" at the end of the rc.local entry be? Well it starts the retrogame program and doesn't continue to boot.
After that I installed ubuntu to usb and edited the sd card in ubuntu...which took a few minutes and worked perfectly well. I could get the roms in that way too.
The good news
The PCB board works perfectly, roms are starting and everything is well!
The bad news
My layout doesn't work. The PI processor is directly over the display connector. The longer it runs, the worse the display gets with a very visible corona of dead pixels and distortions (after cooling down, it looks perfect again luckily).
If anyone has an idea what to do now...I can place it differently but then I will have to resolder most cables. I have some room between connector and processor, so maybe a heatsink and a fan?
