modify: Add an extra function user_local_modify for end users to populate.

Changelog: Title
Signed-off-by: Drew Moseley <drew.moseley@northern.tech>

mender-convert-modify

@@ -20,6 +20,10 @@ function platform_modify() {
     true
 }
 
+function user_local_modify() {
+    true
+}
+
 function trap_exit() {
   echo "mender-convert-modify has finished. Cleaning up..."
   sudo umount -f work/boot
@@ -216,6 +220,9 @@ EOF"
 log_info "Performing platform specific modifications (if any)"
 platform_modify
 
+log_info "Performing user/local specific modifications (if any)"
+user_local_modify
+
 for overlay in "${overlays[@]}"; do
   log_info "Applying rootfs overlay: ${overlay}"
   run_and_log_cmd "sudo rsync --archive --keep-dirlinks --verbose ${overlay}/ work/rootfs/"