Howdy,
In your config file you have:
# CONFIG_VGA_BIOS is not set
in coreboot, which will mean that coreboot doesn't setup the display, but in your SeaBIOS config you have:
CONFIG_SEABIOS_VGA_COREBOOT=y
which will try to reuse whatever coreboot has setup. So as you might understand, this causes a bit of a catch 22. :)
Either tell coreboot to run the BIOS *or* set the VGA Hardware Type to "none" in SeaBIOS.
Other common problems are getting the pci id's for your graphics card wrong and using the wrong offset for SeaBIOS to find CBFS (so it can run things like VGA ROM's).
It would be helpful if you had a distro already loaded, as that will try to initialise the display even if the BIOS hasn't, which can be handy (so you can get and see cbmem output to figure out what is wrong)
After all that, my first answer may not be the solution and there may also be something else stopping the system getting as far as running the VGA BIOS.
Hope this helps,