Need to be able to write CHUID file as a whole

[arekinath]

Currently you can only re-write parts of the CHUID file since it's being generated programmatically in the code. This is a problem if you want to use e.g. signed CHUIDs or set other attributes not currently supported.

We should support writing the entire CHUID file as a blob and storing it. The current functionality could be kept simply as a way of generating a default value for it.