Hey Geert,
Geert Stappers wrote:
When I active menu entry 'Search ISOLINUX menu (USB)', which has grub command 'search_isolinux usb', screen goes shortly black and then the main grub menu re-appears. As if the coreboot payload grub wants to tell "nothing found".
..
What to do to make coreboot grub accepting USB image?
There are two ways forward, but neither actually involves coreboot itself:
1. Debug your grub
For this, please get in touch with the grub community, and remember to make clear that you're using grub on coreboot, not grub with a BIOS.
2. Use a SeaBIOS payload instead of grub
SeaBIOS is an open source BIOS implementation that can be built as coreboot payload, is known to boot well from USB and can additionally also boot other coreboot payloads (which you could use to start grub).
Replacing the payload requires changing the contents of the boot flash on your mainboard, possibly to contain a SeaBIOS payload that you built yourself. This alternative may or may not be practical for you.
In any case, coreboot is not involved in booting the operating system and that is on purpose. This task lies with the payload, so once the payload is running successfully you have to investigate the payload, not coreboot. I hope that makes sense. :)
Kind regards
//Peter