Using the instructions in the old Wiki page  I have used the UEFI method and UEFI tool to extract the video BIOS from the standard binary that comes with the board.
I have followed the layout of some of the other Kconfig files in the mainboard directory and added the following to my boards Kconfig:
config BOARD_SPECIFIC_OPTIONS ... select VGA_BIOS ...
config VGA_BIOS_ID string default "8086,0406"
config VGA_BIOS_FILE string default "pci8086,0406.rom"
When I try to build with that I get an error:
make: *** No rule to make target 'pci8086,0406.rom', needed by 'build/coreboot.pre'. Stop.
The other mainboards that use this methodology all seem to have “select GFX_GMA_PANEL_1_ON_LVDS” but as my display is connected directly to the eDP port I added “select GFX_GMA_PANEL_1_ON_EDP”. This in turn appears to need “select GFX_GMA”. Now when I try to build I get:
warning: (BOARD_SPECIFIC_OPTIONS) selects GFX_GMA which has unmet direct dependencies ((NORTHBRIDGE_INTEL_GM45 || NORTHBRIDGE_INTEL_X4X || NORTHBRIDGE_INTEL_IRONLAKE || NORTHBRIDGE_INTEL_SANDYBRIDGE || NORTHBRIDGE_INTEL_HASWELL || SOC_INTEL_BROADWELL || SOC_INTEL_SKYLAKE || SOC_INTEL_APOLLOLAKE || SOC_INTEL_KABYLAKE || SOC_INTEL_COFFEELAKE || SOC_INTEL_WHISKEYLAKE) && (MAINBOARD_USE_LIBGFXINIT || INTEL_GMA_LIBGFXINIT_EDID))
The board I am working with uses Comet Lake so I have "select SOC_INTEL_COMETLAKE” in the board specific options.
Am I going about this the wrong way or does the depends on list for GFX_GMA in src/drivers/intel/gma/Kconfig need SOC_INTEL_COMETLAKE adding to the list?