arm: mach-k3: security: Allow signing bypass if type is HS-FS

On HS-FS devices signing boot images is optional. To ease use
we check if we are HS-FS and if no certificate is attached
to the image we skip the authentication step with a warning
that this will fail when the device is set to security enforcing.

Signed-off-by: Andrew Davis <afd@ti.com>