Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

Add a description of how to implement verified boot using signed FIT images,
and a simple test which verifies operation on sandbox.

The test signs a FIT image and verifies it, then signs a FIT configuration
and verifies it. Then it corrupts the signature to check that this is
detected.

Signed-off-by: Simon Glass <sjg@chromium.org>