goldfish: Add specification for Goldfish devices.

This adds a new document to properly specified the goldfish
virtual harware platform. This shall help for the following
cases:

  - Porting goldfish support to upstream QEMU.
  - Updating goldfish kernel sources to more recent Linux tree.

Change-Id: Icbf57fa7f3667bfb8e0785f6137d7d79db313b0e
Merge "goldfish: Add specification for Goldfish devices."
Documentation update.

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
Merge "Documentation update."
Merge remote-tracking branch 'aosp/master' into next/aosp/master