Simplify PNOR assembly process

[ghost]

The process of going from compiled firmware components to a PNOR image that you can flash is overly complicated, parts of the tooling isn't maintained, and things are very error prone when changing it.

We could probably do something with ffspart like Cyril prototyped, and I believe @sammj is trying to get cleaned up.

This should improve build time too.

[sammj]

Hey I was just about to create this issue :D ffspart is correct, I'm working on that at the moment, slowly integrating it in fully and removing some leftover dependencies. Most likely we'll get more isolated items such as https://github.com/open-power/pnor/pull/97 in first; once everything is updated to deal with that, ffspart should slot in nicely.