Adding deb/postrm to recreate /opt if it is removed by dpkg

Change-Id: Ic52fd90ab4726c88e395da3621f392d5155866d8
diff --git a/tools/package/deb/postrm b/tools/package/deb/postrm
new file mode 100755
index 0000000..90dd616
--- /dev/null
+++ b/tools/package/deb/postrm
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+# FIXME dpkg will remove /opt if it is empty after removing onos
+#       We should recreate /opt if it was deleted.
+sudo mkdir -p /opt