Move build/build_config_functions.h to sandbox/

There is not much support for build/build_config_functions.h,
as highlighted in http://goo.gl/3ufXOJ. Move it away.

R=jww@chromium.org

Review URL: https://codereview.chromium.org/106163008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239102 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239102