Fix broken verify functionality in crc32 command

Introduced in change d20a40de
"Roll crc32 into hash infrastructure"

The crc32 command with no -v expects an optional 3rd argument to be an
address to store the result in. With the -v switch, the last argument
is a crc, not an address. In the case where -v is set, we should set the
HASH_FLAG_ENV flag since that will first look for the value to be a
digest value, which matches the expected API for the crc32 command.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>