And allow to disable the debug-tweaks by using kas/production.yml.

Kas doesn't support referencing env variables as we assumed it would.
Rewrite the config to take account of this.

In order to support multiple Yocto releases and switch between them,
use an env variable YOCTO_SERIES to set the default refspec. This
value is also passed to Bitbake in the BASE_SERIES variable.

With this we can create series conf files like dunfell.yml to select
a certain Yocto base release/series.