Ok, that will be kind of PDK. But you see, it will never happen. Unless Purism fails to deliver transparent platform and joins the horde of libhybris-based platforms.
The reason for that is exactly that - purism attempts to build a hardware on which you can just run the distro. i.e. it should be supported by mainline kernel with no requirements to inject vendor-specific binary blob for HAL emulation.
So build instructions should be something simple like - use this device-tree table, add these udev rules, blacklist these modules, pre-load those modules, add these kernel boot params - and that’s it.