Describe how to do initial config in testenv

This is really a broader cleanup and revamp of the example Makefiles, adding various examples and explanations.

I was about to add a third make target, testenv-post-start, which one could use to run initial configuration of the testenv (that is only supposed to be run once) but ultimately decided it is better to just do this from testenv-start-extra as we keep down the amount of different targets we have and this just makes it easier to look at IMHO.

Closes #119 (closed)