Thanks Andrey,  that was what I mean.
I tried as your suggestion: flashed an 8 MB Intel-provided image, read back from SPI chip right after flashing, got a 16 MB file (since SPI chip is 16 MB size).
The first 8 MB of 2 files are exactly the same. So, flashing should work fine.

I also went through coreboot mailing list and saw this thread: https://www.coreboot.org/pipermail/coreboot/2017-February/083413.html 
The guy here got the same problem with me:

> Currently: 
> DS6B1 : Green ON 
> DS6B2 : Green ON 
> DS2C1 : OFF 
DS3B1 : RED ON 
>
> No Output on serial console or HDMI.

And you commented:

Red light is never a good sign. Do you have a way to know if PLTRST# has
been de-asserted? 
 
Another thing is that when you stitch with FIT you need to turn off boot
guard completely. I suspect you have it on and that makes CSE to check
signatures. Please check that in Platform protection -> Bootguard
configuration it is set to "bootguard profile 0 - legacy". Then try
restitching fit image and rebuilding coreboot again. 
 
- Andrey

In my case:
PLTRST#  has not been de-asserted. I plugged a PostCode display module in the CRB and RST LED is always Green ON.
- Bootguard configuration is already set to  "bootguard profile 0 - legacy".

Could you please give me further advice? 
Thanks a lot.