EmbeddedPkg: add DT platform driver to select between DT and ACPI

As a follow up to the changes proposed by Laszlo to make ACPI and DT
mutually exclusive on ArmVirtQemu, this patch proposes a DT platform
DXE driver that either installs the NULL protocol PlatformHasAcpiGuid,
or installs the FV embedded DTB binary as a configuration table under
the appropriate GUID, depending on a preference setting recorded as
a UEFI variable, and configurable via a HII screen.

The DTB binary can be embedded in the firmware image by adding the
following to the platform .fdf file:

  FILE FREEFORM = 25462CDA-221F-47DF-AC1D-259CFAA4E326 {
    SECTION RAW = SomePkg/path/to/foo.dtb
  }

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

EmbeddedPkg/Drivers/DtPlatformDxe/DtPlatformDxe.inf

@@ -0,0 +1,58 @@
+## @file
+#
+#  Copyright (c) 2017, Linaro, Ltd. All rights reserved.<BR>
+#
+#  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.
+#
+##
+
+[Defines]
+  INF_VERSION               = 0x00010019
+  BASE_NAME                 = DtPlatformDxe
+  FILE_GUID                 = FC097B3C-2EBD-4A75-A3DA-121DCAB365CC
+  MODULE_TYPE               = DXE_DRIVER
+  VERSION_STRING            = 1.0
+  ENTRY_POINT               = DtPlatformDxeEntryPoint
+
+#
+# The following information is for reference only and not required by the build tools.
+#
+#  VALID_ARCHITECTURES      = IA32 X64 ARM AARCH64
+#
+
+[Sources]
+  DtPlatformDxe.c
+  DtPlatformHii.vfr
+  DtPlatformHii.uni
+
+[Packages]
+  EmbeddedPkg/EmbeddedPkg.dec
+  MdePkg/MdePkg.dec
+  MdeModulePkg/MdeModulePkg.dec
+
+[LibraryClasses]
+  BaseLib
+  DebugLib
+  DxeServicesLib
+  HiiLib
+  MemoryAllocationLib
+  UefiBootServicesTableLib
+  UefiDriverEntryPoint
+  UefiRuntimeServicesTableLib
+
+[Guids]
+  gDtPlatformFormSetGuid
+  gDtPlatformDefaultDtbFileGuid
+  gEdkiiPlatformHasAcpiGuid
+  gFdtTableGuid
+
+[Depex]
+  gEfiVariableArchProtocolGuid        AND
+  gEfiVariableWriteArchProtocolGuid