asymmetric keys: allow pass digest to private_key_generate_signature

This patch add a new function paramter to generate_signature(), the name
is hash. Set hash parameter to true means rsa.c will generate digest by the
hash_also algorithm, set to false means M is digest doesn't need hash.
Caller still need pass hash algorithm to generate_signature() for indicate
what hash algorithm does this digest used.

Signed-off-by: Lee, Chun-Yi <jlee@suse.com>