Add top level build script for building packages.

ports/libtommath/build.sh

Index: ports/libtommath/build.sh
diff --git a/ports/libtommath/build.sh b/ports/libtommath/build.sh
old mode 100755
new mode 100644
index e5a97acc89be3abf65eee98b7bcf3862c6cc6ee8..e9f5f8caa0a3e92171fa3b5904f48b9209da6d3c
--- a/ports/libtommath/build.sh
+++ b/ports/libtommath/build.sh
@@ -3,16 +3,10 @@
 # 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() {
-  MAKE_TARGETS=mtest
-  export EXEEXT=${NACL_EXEEXT}
-  DefaultBuildStep
-}
 
+BUILD_DIR=${SRC_DIR}
+MAKE_TARGETS=mtest
+export EXEEXT=${NACL_EXEEXT}
 
 #TestStep() {
   # To run tests, pipe mtest.nexe output into test.nexe input
@@ -22,14 +16,9 @@ BuildStep() {
   #RunSelLdrCommand mtest/mtest.nexe | RunSelLdrCommand test.nexe
 #}
 
-
 InstallStep() {
   Banner "Installing"
   # copy libs and headers manually
   LogExecute cp *.h ${NACLPORTS_INCLUDE}
   LogExecute cp *.a ${NACLPORTS_LIBDIR}
 }
-
-
-PackageInstall
-exit 0