mkimage: Add support for signing with pkcs11

Add support for signing with the pkcs11 engine. This allows FIT images
to be signed with keys securely stored on a smartcard, hardware security
module, etc without exposing the keys.

Support for other engines can be added in the future by modifying
rsa_engine_get_pub_key() and rsa_engine_get_priv_key() to construct
correct key_id strings.

Signed-off-by: George McCollister <george.mccollister@gmail.com>