Add empty folder for HW_UID

[sperok]

Expose the HW_UID as an empty directory entry

[sperok]

prefix with "UID-", Create contemporaneously with writing HW_UID to config.json

[MiclFierro]

added in dev branch:
In boot.py a folder with name "0x[uid]" is created and any other files/directories named in hex are removed
Setting uid in config has also been moved from utils.py to boot.py to reduce necessary reboots and added a validation step to overwrite erroneous entries.

code review and release expected by eod