Custom MAC address

[troglobit]

Description

• Replace phys-address deviation with new custom-phys-address,
  For details, see #680
• Add syntax migration for existing startup-config and factory-config files
• Add custom-phys-address examples to documentation

Other information

Update documentation, networking.md:

• Add missing VLAN brick to networking LEGO overview
• Add description of brick relationships
• Add section for general interface settiing

Checklist

Tick relevant boxes, this PR is-a or has-a:

• [ ] Bugfix
  • [ ] Regression tests
  • [ ] ChangeLog updates (for next release)
• [X] Feature
  • [X] YANG model change => revision updated?
  • [X] Regression tests added/updated?
  • [X] ChangeLog updates (for next release)
  • [X] Documentation added?
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (please detail in commit messages)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):