Fixes and improvements

New features:

• Move project configuration from configuration.mk to [default-]config.json
  • Project dependencies are resolved automatically
  • There is no "list of projects", so "global" make targets have slightly different meaning, check updated README.md.
• Implement support for data packages, closes #13 (closed)

Fixes:

• Workaround bug in Gaudi CMake (fixes ninja: multiple rules generate ../__init__.py)
• Relax git version requirement
• Do not build Gaudi OpenCL examples, fixes #14 (closed)
• Update distcc version
  • Rebased on upstream master fd943af
  • Fix include server for Online, fixes #18 (closed)
• Fix check for reachability of distcc servers

Improvements:

• Improve automatic stack update with setup.py
• Pin Gaudi version to v33r1
• Pin LbEnv version and cache it between runs
• Quieter checkout of dependencies
• Remove unsupported make targets (use-git-*, *-purge)
• Documentation
  • Explain some config settings, addresses part of #12
  • Update known issues in README
  • Explain how to develop lb-stack-setup (test changes)
  • Add system libraries and headers to prerequisites.md

Internal changes:

• Set distcc timeout explicitly to 5 min (the default anyway)
• Print git version in CI setup job
• Fix cache path in CI make job
• Support giving multiple targets to make.sh
• Make debugging a bit easier
• Reduce number of calls to config.py
• Move checkout from Makefile into setup-make.py