build_tools/make_version: Get MAINLINE_BRANCH from .gitreview.

Use .gitreview defaultbranch setting to determine the mainline branch.
This allows the script to be used against other directories which might
not be on the same defaultbranch.  This can be used by CI scripts to
report the testsuite version being used:
./build_tools/make_version ${TESTSUITE_DIR}

Change-Id: Ifdad4a9d8a26138c41bc6b630ecc3e34ea1c2758

build_tools/make_version

@@ -3,7 +3,6 @@
 AWK=${AWK:-awk}
 GIT=${GIT:-git}
 GREP=${GREP:-grep}
-MAINLINE_BRANCH=16
 
 if [ -f ${1}/.version ]; then
     cat ${1}/.version
@@ -101,6 +100,7 @@ elif [ -d ${1}/.git ]; then
     MODIFIED=""
     SVN_REV=`${GIT} log --pretty=full -1 | ${GREP} -F "git-svn-id:" | sed -e "s/.*\@\([^\s]*\)\s.*/\1/g"`
     if [ -z "$SVN_REV" ]; then
+        MAINLINE_BRANCH=$(git config -f .gitreview --get gerrit.defaultbranch)
         VERSION=`${GIT} describe --long --always --tags --dirty=M 2> /dev/null`
         if [ $? -ne 0 ]; then
             if [ "`${GIT} ls-files -m | wc -l`" != "0" ]; then