hash: Use lower case for hash algorithm names

Rather than use strcasecmp() in the hash algorithm search, require the
caller to do this first. Most of U-Boot can use lower case anyway, and
the hash command can convert to lower case before calling hash_command().
This saves needing strcasecmp() for boards that use hashing but not
the hash command.

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