I believe there are a few 100% FOSS laptops out there that aren't listed on the coreboot site but my age betrays me and I am forgetting what they are called. (I say again purism is a scam, do not buy from them.)
All Chromebooks based on Nvidia and Rockchip SoCs are 100% FOSS as far as firmware goes (graphics acceleration is a different story, but you can run them with software rendering). (Mediatek Chromebooks are 99.9% FOSS, they just have a tiny power management controller with openly available binary firmware.)
As for OP's question, coreboot can not just support a random motherboard out of the box, you will always need code for it. You might be able to write it yourself if you know (or are willing to figure out) enough about how it's laid out. The existing Braswell code will certainly provide you a good stepping stone... board support is usually a tiny amount of work compared to SoC support.
I don't know enough about Intel to tell you whether your board is using BootGuard or how you would find that out, though. If it does, you're probably out of luck. (If it doesn't, it's true that you still need blobs... but you can usually extract these from your vendor firmware and work them into a coreboot image.)