Here we go...
I guess cbmem.log should be enough?
Attached one boot with CONFIG_MAINBOARD_DO_NATIVE_VGA_INIT=y set and one with CONFIG_VGA_ROM_RUN=y and CONFIG_S3_VGA_ROM_RUN=y..
MAINBOARD_DO_NATIVE_VGA_INIT does nothing - display is initialzed as recently as kernel is already loaded and display driver starts working.
VGA_ROM_RUN at least corectly initialize the display, but - as already mentioned - no output from coreboot nor seabios is displayed - just a white blinking cursor until kernel gets loaded.. I guess i have no timeout in grub2 - so this might be the reason why i also see no output from grub..
best regards,
n3ph