efi_loader: image_loader: support image authentication

With this commit, image validation can be enforced, as UEFI specification
section 32.5 describes, if CONFIG_EFI_SECURE_BOOT is enabled.

Currently we support
* authentication based on db and dbx,
  so dbx-validated image will always be rejected.
* following signature types:
    EFI_CERT_SHA256_GUID (SHA256 digest for unsigned images)
    EFI_CERT_X509_GUID (x509 certificate for signed images)
Timestamp-based certificate revocation is not supported here.

Internally, authentication data is stored in one of certificates tables
of PE image (See efi_image_parse()) and will be verified by
efi_image_authenticate() before loading a given image.

It seems that UEFI specification defines the verification process
in a bit ambiguous way. I tried to implement it as closely to as
EDK2 does.

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