imx: hab: Make imx_hab_is_enabled dependent on FIELD_RETURN

imx: hab: Make imx_hab_is_enabled dependent on FIELD_RETURN

The decision on whether HAB is enabled is solely based on the SEC_CONFIG
fuse. The HAB FIELD_RETURN feature is able to permanently disable HAB on
a CPU, after which it is able to boot unsigned firmware. U-Boot however
does not take into account the FIELD_RETURN mode, and refuses to boot
unsigned software when the feature is enabled.

Also take the FIELD_RETURN fuse into account when deciding whether HAB
is enabled. When The FIELD_RETURN fuse is blown, HAB is not enabled.

Tested on i.MX8M Mini, i.MX8M Plus, i.MX8M Nano and i.MX6ULL

Signed-off-by: Paul Geurts <paul.geurts@prodrive-technologies.com>