I'm not sure if you were able to resolve your concerns by following the link which Carlos provided above but, if not, perhaps we here in Ircona (Ircona Home) should be able to help you.
Ircona have been providing BIOS / EFI customization services for Intel based products for over 15 years and have supported Intel's introduction of first BLDK and now FSP/CoreBoot (Embedded) in their Atom class processors. We also have a very experienced Hardware design team which could assist you in the development of your motherboard.
It is clear that the FSP/CoreBoot approach is the correct approach for many designs but it is useful to have a clear understanding of the full software feature set you wish to implement prior to the commencement of your project. We have found that, sometimes, customers end up investing too much effort in trying to achieve some later functionality via coreboot which would have been straightforward with a BIOS implementation. As Ircona have expertise in both approaches, we can assist you in making the right choice at the start thereby minimizing your overall investment.
I look forward to hearing from you and can assure you that you will benefit from engaging my team
Sorry I didn't see this sooner. As someone who went through this pain 18 months ago I will make this recommendation: put the TXE image in there. You hopefully put an XDP connection on your hardware and if so it will not work without it. The JTAG chain will see the uncore device but not the x86. You do not need to sign the images or create a manifest. You need the TXE package anyway to get the Flash Image Tool which you need to set all the boot flags. I use the Slim version: 543572_BYT_I_SEC_SlimFW_PV_RELEASE. You will need to contact your FAE for it as I don't think it is available off the web site any more.
And a heads up: granted I haven't spent much time on it but the latest coreboot is giving me an error on fsp_init. If I go back to my original coreboot from last year it works fine. And this is with the same FSP binaries linked in.
Another possibility is a reference coreboot zip file that you can download if you have an Intel account. I wanted to baseline to it but whomever zipped it up converted tabs to spaces, or vice-versa. Either way I get an error from make on it when I do a 'make menuconfig'.
Good luck with your project. If you have issues post them here and if it's something I also encountered I will let you know what I did.