This commit changes the way build-manifest is invoked, to let it accept
the Rust version directly instead of requiring the path of the Rust
monorepo and letting build-manifest figure out the path on its own.
This allows to run build-manifest without a clone of the monorepo.
- Suggest `x.py setup` if config.toml doesn't exist yet (twice, once
before and once after the build)
- Prompt for a profile if not given on the command line
- Print the configuration file that will be used
- Print helpful starting commands after setup
- Link to the dev-guide after finishing
- Note that distro maintainers will see the changelog warning
