This improves the build documentation beyond just packaging:
- Explain how the configure.py step works
- Explain how to build just executables and tests (for development)
- Explain how to build for specific build mode if you didn't specify a
build mode in configure.py step
- Fix build artifact locations, missing .debs, and add executables and
tests
Message-Id: <20200904084443.495137-1-penberg@iki.fi>