WIP: storing update data after update

[renatav]

Description (e.g. "Related to ...", etc.)

This work covers creation of files containing the update data. Needs to be signed. I would do that using the features that are proposed to be added in #594 once it is merged in.
Whoever continues this work, feel free to push to use this branch.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)