lib: rsa: rsa-verify: don't look for keys in the FIT image

lib: rsa: rsa-verify: don't look for keys in the FIT image

In the function rsa_verify_hash, if the "main" key doesn't
work, u-boot try others keys. But it searches those keys
in the FIT image instead of the u-boot device tree.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>