Provide a way to start from a specific stage from scratch

[Smjert]

Right now if the user want to start from a specific stage (lets say rebuilding from scratch only the final toolchain), it has to manually delete the final folder, which is the install folder, and also the llvm/build-llvm-final. Having something that automatically does that would be nicer.

Ideally this option is only needed when the user is fiddling with the configuration or has to fix some bugs, otherwise everything should just work as is. But having an option to do so improves developing time.