image-tools: T6176: addendum to fixes for boot console

[jestabro]

Change Summary

Minor additions to PRs 3443/3444:

(1) add line dropped during rebase to use the defined function console_hint() as prompt default
(2) when dropping ttyUSB menu entries, prune instead of ignore, to avoid a duplicated entry in original legacy install

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):

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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

[jestabro]

@Mergifyio backport sagitta

[mergify]

backport sagitta

✅ Backports have been created

• #3446 image-tools: T6176: addendum to fixes for boot console (backport #3445) has been created for branch sagitta