generate_medkit: use package's bootstrap ability

Packages now can bootstrap configuration provided from nevironment variables on their own. That removes need to generate configuration files here and makes it overall more general.

Variables are renamed to explicitly differentiate those participating in packages bootstrap from those only passed to fakeroot trough environment. Those are also variable names expected by package them self.

Companion MR https://gitlab.nic.cz/turris/turris-os-packages/-/merge_requests/500