build: T3664: clone vyos-1x under build dir instead of as submodule

[jestabro]

Change Summary

This drops the use of a git submodule to manage the import of the vyos-1x python module for scripts/image-build/build-vyos-image, instead cloning by branch name under the build directory. Note that the argument to git.checkout can take a tag or commit hash in place of the branch name.

In addition, some minor reorganization allows (1) restoring the vyos-build git repo check to be returned to its original place before the changes to accommodate submodules (2) running basic checks earlier in the script in order to fail faster (3) earlier loading of defaults.toml for branch name, and later addition of vyos-1x tag/commit hash, if needed.

This replaces the obsoleted #583 which will be closed.

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 use of submodule in favor of clone of vyos-1x.

Related Task(s)

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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