This allows snippet generators to explicitly order entries: any string
can be set as an entry's "sort key". If set, sd-boot will use it to sort
entries on display.
New logic is hence (ignore the boot counting logic)
sort-key is set → primary sort key: sort-key (lexicographically increasing order)
→ secondary sort key: machine-id (also increasing order)
→ tertiary sort key: version (lexicographically decreasing order!)
sort-key is not set → primary sort key: entry filename (aka id), lexicographically increasing order)
With this scheme we can order OSes by their names from A-Z but then put
within the same OS still the newest version first. This should clean up
the order to match expectations more.
Based on discussions here:
https://github.com/systemd/systemd/pull/22391#issuecomment-1040092633
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