gen: Add progressive hash API

Add hash_init(), hash_update() and hash_finish() to the
hash_algo struct. Add hash_lookup_algo() to look up the
struct given an algorithm name.

Signed-off-by: Hung-ying Tyan <tyanh@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>