making coreboot work on device is anything but trivial and involves quite some work from experienced people. How much work this actually is depends on many factors, but mostly on the motherboard chipset used. Doing this without help from the manufacturer is even more complicated as this ends up in a lot of trial and error work. Honestly, it's simply very hard to do such an estimation without doing some part of the actual work e.g., investigating the platform and finding out whats already there to start with. Afterwards one might be able to do a proper estimation of the expected work.