image-tools: T6176: fixes for boot console (backport #3443)

[mergify]

Change Summary

Several fixes for image-tools boot console:
(1) drop ttyUSB as unuseful, as discussed
(2) add a minor utility to set boot console: this solves a chicken-egg issue if upgrading, in compatibility mode, from an image before the fix in https://vyos.dev/T6168
(3) attempt to provide console-type prompt hint during 'install image'

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):
  drop unused/unuseful usb boot console

Related Task(s)

• https://vyos.dev/T6327
• https://vyos.dev/T6184
• https://vyos.dev/T6278

Related PR(s)

Component(s) name

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

---

This is an automatic backport of pull request #3443 done by [Mergify](https://mergify.com).