hash.c: Conditionally compile hash_command, static hash_show

The function hash_show is now only called by hash_command, so mark it as
static (and drop from hash.h).  We only call hash_command when any of
CONFIG_CMD_CRC32, CONFIG_CMD_SHA1SUM or CONFIG_CMD_HASH are set.  Since
hash.c is linked in unconditionally we must take extra care with
functions that bring in read-only strings as these will not be
discarded.

Signed-off-by: Tom Rini <trini@konsulko.com>