overlay: Print error message to stderr

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ie9f40997bc6a87ef4eb8f157e9b764a2b2e6a641
diff --git a/overlay/overlay b/overlay/overlay
index 8c3ac99..a32f218 100755
--- a/overlay/overlay
+++ b/overlay/overlay
@@ -27,7 +27,7 @@
     umount $o_lowerdir
     if [ -z "$o_options" ]
     then
-        echo $o_lowerdir is not an overlay
+        echo >&2 $o_lowerdir is not an overlay
         exit 1
     else
         echo "${o_options}" |