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

Test: repo upload w/ and wo/ changes in mainline path.
Change-Id: I556edf6cb6bee549c17bc9ba559a2c107df4280d
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