commands: add md5/sha1/sha256sum commands using the digest api ...

The interface emulates the Linux commands, except that you can
specify a sub area - E.G.:

barebox:/ md5sum /dev/fd0 2M+1M /env/config /env/bin/boot 10+2
61c4c0180b044191d28f27545f43562f  /dev/fd0	0x00200000 ... 0x00300000
908b84bcbadd2f263583a65ff31d1cad  /env/config	0x00000000 ... 0x000003a7
f23bd15825cc5006cf5f9fd486d82d2d  /env/bin/boot	0x0000000a ... 0x0000000c

Adds around 1400 bytes (+ size of digest code) with everything enabled.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>