Add a repo upload hook for checking the mainline code change.

Test: repo upload w/ and wo/ changes in mainline path.
Change-Id: If7873924ba4a36b84e9c9df852b5f6d70fd0b13f
diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
new file mode 100644
index 0000000..86b6de3
--- /dev/null
+++ b/PREUPLOAD.cfg
@@ -0,0 +1,2 @@
+[Hook Scripts]
+mainline_hook = ${REPO_ROOT}/frameworks/av/tools/mainline_hook.sh