The Thinkpad x140e has an AMD 16h family chip codenamed “Kabini”. These chips have a PSP despite the 15h.org landing page statement:

…“It enforces no firmware signing (allowing things such as coreboot), can be booted entirely with free firmware (including memory initialization), and it requires no privileged management coprocessor (such as the Platform Security Processor or Management Engine, found on newer AMD and Intel platforms respectively).”…

How is it that this 16h machine can run coreboot-15h? My theory is that perhaps the PSP in the 16h chips was so new that it did not yet become a dictator of what firmware may boot. But I would rather not guess.