You can see the available documentation for the i.MX6UL on the following link. Under Reference Manual you will see the Security Reference Manual for the i.MX6UL. You can request this document by clicking on it. However, since this is a moderated document you will need an NDA to access it. I would recommend contacting your NXP Distributor and explaining you need access to this document so they can help you signing the NDA.
This Security Refence Manual should cover the specifics on the Security Features of the i.MX6UL. I would also recommend reading Application Note 4581 which covers secure boot.
I hope this helps,