OvmfPkg: add library class BlobVerifierLib with null implementation

BlobVerifierLib will be used to verify blobs fetching them from QEMU's
firmware config (fw_cfg) in platforms that enable such verification.

The null implementation BlobVerifierLibNull treats all blobs as valid.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3457
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

OvmfPkg/Library/BlobVerifierLibNull/BlobVerifierNull.c

@@ -0,0 +1,33 @@
+/** @file
+
+  Null implementation of the blob verifier library.
+
+  Copyright (C) 2021, IBM Corporation
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include <Library/BaseLib.h>
+#include <Library/BlobVerifierLib.h>
+
+/**
+  Verify blob from an external source.
+
+  @param[in] BlobName           The name of the blob
+  @param[in] Buf                The data of the blob
+  @param[in] BufSize            The size of the blob in bytes
+
+  @retval EFI_SUCCESS           The blob was verified successfully.
+  @retval EFI_ACCESS_DENIED     The blob could not be verified, and therefore
+                                should be considered non-secure.
+**/
+EFI_STATUS
+EFIAPI
+VerifyBlob (
+  IN  CONST CHAR16    *BlobName,
+  IN  CONST VOID      *Buf,
+  IN  UINT32          BufSize
+  )
+{
+  return EFI_SUCCESS;
+}