boot: Only define checksum algos when the hashes are enabled

Don't define checksum algos when the underlying hashes are not enabled.
This allows disabling these hashes in SPL (or U-Boot).

Fixes: d16b38f4 ("Add support for SHA384 and SHA512")
Fixes: 646257d1 ("rsa: add sha256-rsa2048 algorithm")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>