Too many sources of default configuration

On more than one occasion we were confused about what the default value for a particular option is. This is because we have multiple places where such defaults are defined, which can have diverging and conflicting defaults.

1. We have default values defined in code, e.g. in modules/ksb/BuildContext.pm
2. We have kdesrc-buildrc-kf5-sample and kdesrc-buildrc-kf5-sample
3. kdesrc-build --initial-setup generates a kdesrc-buildrc file which comes with its own defaults
4. kdesrc-build-setup is another way to creates a kdesrc-buildrc

That's too many places to keep in sync and ways to go wrong. See for example !297 (merged)