Add support for unattended updates

[saba8814]

This commit introduces unattended-update cron consumer. Unattended updates are configurable to perform update autonomously or to wait for admin to reboot. Unattended upadate reuses UPDATE URL(pointing to Atom/RSS feed) from check-update and is configurable under system/software yang. Cron consumer runs rauc install .pkg thus HTTP/s range shall be supported on the web server. Also docs are included and migration script due to changes in update url. Original PR: #1562
Resolves: #1301

Description

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):

[troglobit]

Nice work on this!

Lots of minor things still to change and discuss with other team members.

[saba8814]

2. Re‑installs the same bundle on every occurrence until reboot (medium, robustness)
   With reboot manual, running VERSION never changes after a successful install, so each schedule tick downloads and re‑flashes the identical bundle to the inactive slot. Dev builds (IS_RELEASE=false) do this unconditionally. That is needless flash wear and bandwidth, and it makes the lock the only thing preventing overlap. Check the inactive slot's version via rauc status --output-format=json (or remember the last installed tag) and skip when it already matches LATEST_TAG.