[crypto] Add optional signing capabilities to crypto::rsa
[crypto] Add optional signing capabilities to crypto::rsa

This change adds signing capabilities to crypto::rsa. While Cerberus
itself does not need to be able to create signatures, an eventual
manifest tool will, and making the interface for signing fit with the
interface for RSA operations makes more sense than crypto::ring's ad-hoc
interface.

Signed-off-by: Miguel Young de la Sota <71b8e7f4945fd97b98544cf897992af89646547a@google.com>

lowRISC/manticore,lowRISC/manticore,lowRISC/manticore