build: T7109: add support for includes_binary in flavors

[dmbaturin]

Change summary

Adds includes_binary option to flavor files, analogous to includes_chroot but for files that will go to data/live-build-config/includes.binary.

That can be used to modifying boot settings — for example, changing the boot splash for customized images.

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)

Related PR(s)

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

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