In barebox the function digest_alloc() allocates a digest based on a string.
When a subsystem already uses an integer value to identify a digest it makes no
sense to create a string and pass it to digest_alloc(), where it is parsed
again. This patch adds the possibility to get a digest by an enum.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>