[th/envsetup-merge] simplify envsetup.sh (part 1)
I think envsetup.sh is problematic, it should significantly improve.
In particular, because I want to run NM-ci in containers... which actually works just fine, if you first create a managed eth0. Optimally, that requirement can be lifted, which is what I want to do.
This is a first start at that.
The most important first step is to drop the prepare/envsetup/*
files, because reworking the code when it's split out in 8 files is not possible. As such, this is reopening !1059 (closed) and !1075 (closed).