dlx: enable 'footer' style encryption

* add 'length' parameter for /data partition
* change to 'footer' style encryption

Change-Id: Id343bfc8cc47b8661b708be97f57098e612912a5
dlx: use m7 kernel repo

Change-Id: I6c1dd71f0f101b24a709400dc0888ed8d6062e3a