Makebuildserver for debian jenkins

@mvdan and I worked out a plan with Holger, the Debian Developer running the reproducible builds jenkins build farm. The idea is for that to just run ./makebuildserver in the jenkins script. Then we'll make sure that it works. This also makes it easy for anyone to set up and run an FDroid build instance, which is important for things like the verification server and for people to be able to run reproducible builds on their own machine.

This does change some of the defaults, but they can all be overridden in the makebs.config.py for f-droid.org.

More comments in the commits.