Thanks for the fast answer. For example i have the issue with an Asrock G41 mainboard. I have tried 2 different AMD cards (3000 series and 7000 series) and 3 different Nvidia cards (old 6000 series, newer 9000 series and even newer gtx 500 series). All those PCIe cards had no issues before to display graphical output at boot time. All those can nott display graphical output at boot time any more since coreboot is been flashed to the SPI chip.
You wrote "this should definitely be bootable". The system is booting, but i dont have graphical grub output any more like i always had before. The screen is black at the time when the grub should be displayed. When the normal 3 seconds grub timeout is done, the system boots.
How can i get the graphical boot functionality on external PCIe GPUs back?