efi_loader: image_loader: add digest-based verification for signed image

In case that a type of certificate in "db" or "dbx" is
EFI_CERT_X509_SHA256_GUID, it is actually not a certificate which contains
a public key for RSA decryption, but a digest of image to be loaded.
If the value matches to a value calculated from a given binary image, it is
granted for loading.

With this patch, common digest check code, which used to be used for
unsigned image verification, will be extracted from
efi_signature_verify_with_sigdb() into efi_signature_lookup_digest(), and
extra step for digest check will be added to efi_image_authenticate().

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>