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This patch is adds support to "native" (i.e. Cygwin-less) Windows
NDK build through the new 'ndk-build.cmd' Windows shell script.

Just call it from the cmd.exe command-line, when in your project
path. The script takes the same arguments than 'ndk-build'.

+ Adds the source of a new tiny Windows-specific command-line
  "echo" program.  The reason for this is that the 'echo' that
  comes with cmd.exe is too weird and will not work properly
  when invoked from make (e.g. it can't deal with double-quoted
  strings properly: it will print the double-quotes in the
  output).

+ build/tools/build-host-toolbox.sh: new script to rebuild the
  'echo' program on Windows.  The idea is that the 'toolbox' may
  contain other command-line programs like "cp" or "mkdir" in
  the future, to avoid relying on the not-so-robust cmd.exe
  versions of these commands (del, md, xcopy), if necessary.

+ Make build-host-prebuilts.sh call build-host-toolbox.sh
  when generating Windows binaries, and package-release.sh
  unpack the corresponding archive when generating the
  Windows packages.

+ build-funcs.sh can now be used to build host binaries too.

+ docs/CHANGES.html: document new native win32 build support.

Change-Id: I4d30f6c6532ae84deb95e7e5f595af76b97d9257