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This patch fixes the ndk-stack tool to properly work as advertized, and
adds a new documentation file (docs/NDK-STACK.html) to illustrate its usage.

+ Add build/tools/build-ndk-stack.sh to automatically rebuild the
  host tool from sources (and from scratch).

+ Ensure that rebuild-all-prebuilt.sh invokes build-ndk-stack.sh
  and places the resulting binary at the top of target NDK directory.

+ Ensure that package-release.sh places the prebuilt binaries into
  the final packages.

+ Add a --host-only option to rebuild-all-prebuilt.sh to indicate
  that you only want to rebuilt the host binaries (and none of the
  target ones). The --darwin-ssh and --mingw options also imply
  --host-only.

Change-Id: I5f26e83378c02bfd3b62fa76a7c50c5755104fa9