image: Add crypto_algo struct for RSA info

Cut down on the repetition of algorithm information by defining separate
checksum and crypto structs. image_sig_algos are now simply pairs of
unique checksum and crypto algos.

Signed-off-by: Andrew Duda <aduda@meraki.com>
Signed-off-by: aduda <aduda@meraki.com>
Reviewed-by: Simon Glass <sjg@chromium.org>