* helm-files.el (helm-ff-auto-update--state): New internal flag.
(helm-ff-delete-char-backward--exit-fn): Use it to control the state of helm-ff-auto-update-flag.
* helm-mode.el (helm-read-file-name): bind helm-ff-auto-update--state.
* helm-files.el: Bind DEL in read-file-name map.