This patch updates the (very dated) documentation on how to build
the emulator. It also tries to explain the current refactoring
process to help third-party contributors send patches to external/qemu.

+ Get rid of obsolete make-distrib.sh / build-emulator.sh scripts since
  they don't work anymore. Currently, we do not support packaging the
  emulator sources into its own tarball that can be easily rebuilt,
  mainly due to dependencies with the GPU emulation libraries that
  can only be built with the platform build (at the moment).

Change-Id: I76a151318d2b925eb916eb4ec9e9aa51b4457672