[digest] Add commands for all enabled digest algorithms

[digest] Add commands for all enabled digest algorithms

Add "sha256sum", "sha512sum", and similar commands.  Include these new
commands only when DIGEST_CMD is enabled in config/general.h and the
corresponding algorithm is enabled in config/crypto.h.

Leave "mdsum" and "sha1sum" included whenever only DIGEST_CMD is
enabled, to avoid potentially breaking backwards compatibility with
builds that disabled MD5 or SHA-1 as a TLS or X.509 digest algorithm,
but would still have expected those commands to be present.

Signed-off-by: Michael Brown <mcb30@ipxe.org>