rsa: Use checksum algorithms from struct hash_algo

Currently the hash functions used in RSA are called directly from the sha1
and sha256 libraries. Change the RSA checksum library to use the progressive
hash API's registered with struct hash_algo. This will allow the checksum
library to use the hardware accelerated progressive hash API's once available.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
CC: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
(Fixed build error in am335x_boneblack_vboot due to duplicate CONFIG_DM)

Change-Id: Ic44279432f88d4e8594c6e94feb1cfcae2443a54