lib: rsa: avoid overriding the object name when already specified

If "object=" is specified in "keydir" when using the pkcs11 engine do
not append another "object=<key-name-hint>". This makes it possible to
use object names other than the key name hint. These two string
identifiers are not necessarily equal.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Bastian Krause <bst@pengutronix.de>
Reviewed-by: George McCollister <george.mccollister@gmail.com>