Rework configuration for `bootstrap_repositories.py`

[Korulag]

Right now our platform configuration that is used by bootstrap_repositories.py is a very long config and it's easy to make errors when editing it. We need to rework it to make modifications simpler:

• Move each platform into a separate config;
• Make configs themselves as our beholder ones: 1 URL per repository with ability to specify architectures to create repositories for.