Don't define checksum algos when the underlying hashes are not enabled.
This allows disabling these hashes in SPL (or U-Boot).
Fixes: d16b38f427 ("Add support for SHA384 and SHA512")
Fixes: 646257d1f4 ("rsa: add sha256-rsa2048 algorithm")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>