asymmetric keys: implement EMSA_PKCS1-v1_5-ENCODE in rsa

Implement EMSA_PKCS1-v1_5-ENCODE [RFC3447 sec 9.2] in rsa.c. It's the
first step of signature generation operation (RSASSA-PKCS1-v1_5-SIGN).

This patch is temporary set emLen to pks->k, and temporary set EM to
pks->S for debugging. We will replace the above values to real signature
after implement RSASP1.

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