EmbeddedPkg: introduce PlatformHasAcpiLib

Add the shorter-term library instance outlined in the previous patch to
EmbeddedPkg, so that we can imbue AcpiTableDxe with a protocol dependency
on EDKII_PLATFORM_HAS_ACPI_GUID.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

EmbeddedPkg/Library/PlatformHasAcpiLib/PlatformHasAcpiLib.c

@@ -0,0 +1,36 @@
+/** @file
+  A hook-in library for MdeModulePkg/Universal/Acpi/AcpiTableDxe.
+
+  Plugging this library instance into AcpiTableDxe makes
+  EFI_ACPI_TABLE_PROTOCOL and (if enabled) EFI_ACPI_SDT_PROTOCOL depend on the
+  platform's dynamic decision whether to expose an ACPI-based hardware
+  description to the operating system.
+
+  Universal and platform specific DXE drivers that produce ACPI tables depend
+  on EFI_ACPI_TABLE_PROTOCOL / EFI_ACPI_SDT_PROTOCOL in turn.
+
+  Copyright (C) 2017, Red Hat, Inc.
+
+  This program and the accompanying materials are licensed and made available
+  under the terms and conditions of the BSD License which accompanies this
+  distribution. The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, WITHOUT
+  WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+**/
+
+#include <Base.h>
+
+RETURN_STATUS
+EFIAPI
+PlatformHasAcpiInitialize (
+  VOID
+  )
+{
+  //
+  // Do nothing, just imbue AcpiTableDxe with a protocol dependency on
+  // EDKII_PLATFORM_HAS_ACPI_GUID.
+  //
+  return RETURN_SUCCESS;
+}