build: T3664: include build flavor name in the version file

[dmbaturin]

Change Summary

Right now, version data doesn't include flavor. If we include it, we can add a warning to add system image when the user tries to upgrade to a non-matching flavor, and also improve the update checker to suggest the correct flavor link.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Component(s) name

Build scripts.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[c-po]

@Mergifyio backport sagitta

[mergify]

backport sagitta

✅ Backports have been created

• #608 build: T3664: include build flavor name in the version file (backport #605) has been created for branch sagitta