Add hash command to perform hashing using various algorithms

This new command supports hashing SHA1 and SHA256. It could be extended
to others such as MD5 and the CRC algorithms. The syntax is modeled on
those:

   hash <algorithm> <address> <length> [*<dest_addr> | <dest_envvar>]

to calculate a hash, and:

   hash -v <algorithm> <address> <length> [*<verify_addr> | <verify_envvar>]

to verify a hash.

Use CONFIG_CMD_HASH to enable the command, CONFIG_SHA1 to enable SHA1 and
CONFIG_SHA256 to enable SHA256.

The existing sha1sum command remains.

Signed-off-by: Simon Glass <sjg@chromium.org>