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