Add top level build script for building packages.

ports/dreadthread/build.sh

 #!/bin/bash
 # Copyright (c) 2011 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
 
 BuildStep() {
   Banner "Building ${PACKAGE_NAME}"
   export PACKAGE_DIR="${NACL_PACKAGES_REPOSITORY}/${PACKAGE_NAME}"
   MakeDir ${PACKAGE_DIR}
   ChangeDir ${PACKAGE_DIR}
   ${NACLCC} -c ${START_DIR}/dread.c -o dread.o
   ${NACLCC} -c ${START_DIR}/dread_chain.c -o dread_chain.o
   ${NACLAR} rcs libdreadthread.a \
       dread.o \
       dread_chain.o
   ${NACLRANLIB} libdreadthread.a
 }
 
 InstallStep() {
   Banner "Installing ${PACKAGE_NAME}"
   export PACKAGE_DIR="${NACL_PACKAGES_REPOSITORY}/${PACKAGE_NAME}"
   cp ${PACKAGE_DIR}/libdreadthread.a ${NACLPORTS_LIBDIR}
   cp ${START_DIR}/dreadthread.h ${NACLPORTS_INCLUDE}
   cp ${START_DIR}/dreadthread_ctxt.h ${NACLPORTS_INCLUDE}
   cp ${START_DIR}/dreadthread_chain.h ${NACLPORTS_INCLUDE}
 }
 
 PackageInstall() {
   PreInstallStep
   BuildStep
   InstallStep
 }
-
-PackageInstall
-exit 0