systemd: change uuid generation

[chewitt]

Using the ethernet MAC to generate a unique identifier is proving unreliable as manufacturers of some cheap ARM devices recycle the same MAC among their boxes and we cannot rely on community builders to always patch their kernels to generate a unique MAC from the CPU serial number. In the modern age of privacy it is also nice to remove the association between the unique identifier and specific user hardware.

This change also forces the machine-id to be 32-chars of lowercase hex to mitigate the "do not track" attempts of some users. A tiny minority zero/null the machine-id. The large majority use a text phrase like "do not track my install!" which due to their different languages and the artistic use of typos, punctuation and/or expletives still provides a uniquely identifiable phrase.

Active install stats for 9.0 will be fuzzy for a while as this rolls out but we age duplicates from the DB after 14d. We also lose persistence of machine-id over reinstalls but the overall percentage of users doing repeat installation on the same device is low. In the longer term we'll also see a small bump in active installs as we reduce the number of devices sharing the same machine-id.

[lrusak]

Why don't we just use systemd-machine-id?

https://www.freedesktop.org/software/systemd/man/systemd-machine-id-setup.html

[chewitt]

I tried it and ran into some weird stuff that looks like systemd doesn't like the symlink relocation from /etc to /storage and might need patching to resolve. This approach also allows content validation of the UUID and removal of the older machine-id.