Add top level build script for building packages.

ports/git/build.sh

 #!/bin/bash
 # Copyright (c) 2013 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source pkg_info
-source ../../build_tools/common.sh
 
 # Do a verbose build so we're confident it's hitting nacl's tools.
 export MAKE_TARGETS="V=1"
 
 export EXTRA_CONFIGURE_ARGS="--prefix= --exec-prefix="
 
 ConfigureStep() {
   local SRC_DIR=${NACL_PACKAGES_REPOSITORY}/${PACKAGE_DIR}
   ChangeDir ${SRC_DIR}
   autoconf
 
-  export NACL_BUILD_DIR=${NACL_PACKAGES_REPOSITORY}/${PACKAGE_DIR}
+  export BUILD_DIR=${NACL_PACKAGES_REPOSITORY}/${PACKAGE_DIR}
   export NACL_CONFIGURE_PATH=./configure
   DefaultConfigureStep
 
   # Git's build doesn't support building outside the source tree.
   # Do a clean to make rebuild after failure predictable.
   LogExecute make clean
 }
 
 InstallStep() {
   MakeDir ${PUBLISH_DIR}
   local ASSEMBLY_DIR="${PUBLISH_DIR}/tar"
 
   export INSTALL_TARGETS="DESTDIR=${ASSEMBLY_DIR} install"
   DefaultInstallStep
 }
-
-PackageInstall
-exit 0