OvmfPkg: add 'initrd' shell command to expose Linux initrd via device path

Add a new 'initrd' command to the UEFI Shell that allows any file that is
accessible to the shell to be registered as the initrd that is returned
when Linux's EFI stub loader invokes the LoadFile2 protocol on its special
vendor media device path.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2564
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/LinuxInitrdDynamicShellCommand/LinuxInitrdDynamicShellCommand.inf

@@ -0,0 +1,53 @@
+##  @file
+# Provides 'initrd' dynamic UEFI shell command to load a Linux initrd
+# via its GUIDed vendor media path
+#
+# Copyright (c) 2020, Arm, Ltd. All rights reserved.<BR>
+#
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+#
+##
+
+[Defines]
+  INF_VERSION                    = 1.27
+  BASE_NAME                      = LinuxInitrdDynamicShellCommand
+  FILE_GUID                      = 2f30da26-f51b-4b6f-85c4-31873c281bca
+  MODULE_TYPE                    = DXE_DRIVER
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = LinuxInitrdDynamicShellCommandEntryPoint
+  UNLOAD_IMAGE                   = LinuxInitrdDynamicShellCommandUnload
+  UEFI_HII_RESOURCE_SECTION      = TRUE
+
+#
+#  VALID_ARCHITECTURES           = IA32 X64 ARM AARCH64 EBC
+#
+
+[Sources.common]
+  LinuxInitrdDynamicShellCommand.c
+  LinuxInitrdDynamicShellCommand.uni
+
+[Packages]
+  MdePkg/MdePkg.dec
+  ShellPkg/ShellPkg.dec
+  MdeModulePkg/MdeModulePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[LibraryClasses]
+  DebugLib
+  DevicePathLib
+  HiiLib
+  MemoryAllocationLib
+  ShellLib
+  UefiBootServicesTableLib
+  UefiDriverEntryPoint
+  UefiHiiServicesLib
+
+[Protocols]
+  gEfiDevicePathProtocolGuid                      ## SOMETIMES_PRODUCES
+  gEfiHiiPackageListProtocolGuid                  ## CONSUMES
+  gEfiLoadFile2ProtocolGuid                       ## SOMETIMES_PRODUCES
+  gEfiShellDynamicCommandProtocolGuid             ## PRODUCES
+
+[DEPEX]
+  TRUE