efi_loader: image_loader: add a missing digest verification for signed PE image

At the last step of PE image authentication, an image's hash value must be
compared with a message digest stored as the content (of SpcPeImageData type)
of pkcs7's contentInfo.

Fixes: commit 4540dabd ("efi_loader: image_loader: support image authentication")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>