So by default if you want to build for HD-8570M, you change in .config (6665 ---> 6663) and you simply complete your build?
Yes, and indeed it could be said that "make menuconfig" is "only a GUI for the .config". If you'd like, you could avoid running it - as long as your manual changes to .config are still compatible with Kconfigs of coreboot source tree (Kconfigs are determining the default values and dependencies of many configuration options) - and if your desired .coreboot configuration is the same as this one which I provided as an example. "6665 ---> 6663" change is very simple - so could be done manually - however for the bigger changes it is more convenient to use a GUI menu, since you may forget about some dependencies and if any of your .config options are in conflict with Kconfigs, these .config options will be reset during a build (there will be a message at the beginning of build but you could skip it accidentally) and there will be a problem. "GUI menu" also helps you to learn more about the coreboot options, you could press a question sign on any of them and get a small useful README.
So if you make a snapshot after applying the patches, you can then change the .config to your preferences and make a build and do that again if you need another build, correct?
Yes, but such a simple switch is possible only if you are building for the same board. If you'd like to build for a different board, you'd also need to run a "make clean" to clean the compiled files, and maybe even "make distclean" if there are any problems. Perhaps it's better to keep the separate coreboot directories for the different boards.
You would use the cbfstool only if you make a snapshot after you already built and then want to change the dgpu, correct?
Not to change the dGPU but rather to add the AtomBIOS for a different one - since the AtomBIOS for another dGPU will still be inside your CBFS (Coreboot ROM's FileSystem), so both dGPUs will be supported - unless you'd remove another AtomBIOS, which is already there, manually (with cbfstool). Also, you could use the cbfstool to add a floppy-based operating system - such as KolibriOS - or simply to print a memory map of your coreboot.rom to see how much of a free space there's left.
Best regards, Mike Banon