Removal of deprecated CONFIG_BOOTM_OPTEE
----------------------------------------

The support for late loading of OP-TEE had been deprecated and ultimately
removed as it greatly increased the attack surface and was only supported
on 32-bit ARM systems.

OP-TEE loading is now only supported
:ref:`in the prebootloader <optee_early_loading>`.

For i.MX6 boards, this can be enabled by enabling
``CONFIG_FIRMWARE_IMX6_OPTEE``.

sha1 no longer accepted for secure boot
---------------------------------------

FIT images using sha1 are no longer accepted as secure boot images. sha1 has
been proven insecure in 2017.