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>

from Linux 3.9

linux generic implementation

$ ls -al build/versatilpb/arch/arm/pbl/zbarebox.bin
-rw-r--r-- 1 root root 210829 Mar 24 13:21 build/versatilpb/arch/arm/pbl/zbarebox.bin

linux arm v4 asm implementation
$ ls -al build/versatilpb/arch/arm/pbl/zbarebox.bin
-rw-r--r-- 1 root root 207786 Mar 24 13:23 build/versatilpb/arch/arm/pbl/zbarebox.bin

we win 3043 bytes and speed cf code

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>