rsa: use new openssl API to create signature

Previous implementation of the rsa signature was using
the openssl API EVP_Sign*, but the new openssl API
EVP_DigestSign* is more flexible. So we move to this
new API.

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