generate-reprepro-codename: Make architectures and (udeb) components configurable

Architectures, components and udeb components are now configurable by either using parameters or environment variables. Parameters will overwrite environment variables.

The udeb components parameter value may be explicitly(!) empty (e.g. --udeb-components "") in which case UDebComponents is not set in the created distribution. If no parameters or environment variables are given, sane defaults are used.

This PR is mainly useful if generate-reprepro-codename is used as a standalone tool. I tried hard to not break backwards compatibility (behavior) by making the parameters optional and default values.