scripts/*: add Makefile generator.

This commit adds a few scripts to the packages repository:

• configure

  Typical 'configure' script; run to see usage.

• scripts/genmake

  Generates a top-level 'Makefile' that will either
  build packages in the correct order, or sleep to
  simulate package builds. The main purpose of this
  tool is to enable concurrent package builds, and
  to provide a framework to analyze build strategies.