utils: T5239: add low-level read from config.boot (RPKI fixes)

[c-po]

Change Summary

Backport from current codebase

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)

• https://vyos.dev/T5239

Related PR(s)

• init: T2044: always start/stop rpki during system boot vyatta-cfg#73

Component(s) name

Util helpers for vyos-router startup

Proposed changes

How to test

Smoketest result

Checklist:

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

[dmbaturin]

I agree with the idea, but is the function really reusable? Maybe place it in the script itself?

[jestabro]

I'll mention that I have had possible uses for the utility function, since initially writing it in this generality --- no actual use yet, but I would prefer to leave the option. Of course, the backport for equuleus could merge it in to the script, but perhaps unnecessary ...