include: image.h: add key info to image_sign_info

For FIT verification, all the properties of a public key come from
"control fdt" pointed to by fdt_blob. In UEFI secure boot, on the other
hand, a public key is located and retrieved from dedicated signature
database stored as UEFI variables.

Added two fields may hold values of a public key if fdt_blob is NULL, and
will be used in rsa_verify_with_pkey() to verify a signature in UEFI
sub-system.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>