build-rootfs: Gracefully handle short version strings

[joelcapitao]

get_treefile() crashes when parsing versions with fewer than 3
components (e.g., "44"). Now we pad the version parts list with
"0" to ensure we always have at least 3 elements.
As a follow-up of e6d3ed2.

[gemini-code-assist]

Code Review

This pull request correctly handles short version strings in get_treefile by padding them to prevent crashes. The approach is sound. I've included a suggestion to make the implementation more concise and Pythonic, which improves code readability.

[dustymabe]

I feel like this needs a heavy comment.

Here is something similar, but might be a little more obvious why the change is needed:

(x, y, _) = version.split('.', 2) if '.' in version else (version, '0', '0')