mdt: Create a default Packages file after install

This fixes an issue with 'apt-get update' calls where it would ignore the local
repo and complain loudly when it "404"s.

Change-Id: I853815704273bacad9e86b68313e1017376847b7
diff --git a/debian/postinst b/debian/postinst
new file mode 100644
index 0000000..23f8e64
--- /dev/null
+++ b/debian/postinst
@@ -0,0 +1,4 @@
+#!/bin/sh
+set -e
+
+touch /var/cache/mdt/packages/Packages