ArmPkg/PlatformBootManagerLib: Add path to boot UEFI Shell over UiApp

The UEFI Shell is a non-active boot option, at the opposite of UiApp.
If no valid boot option is found, UiApp is selected. UiApp requires a
human interaction. When installing a new EDKII image in CIs or when
scripting is required, this is problematic.

If no valid boot option is discovered, add a path to directly go to
the UEFI Shell where the startup.nsh script is automatically executed.
The UEFI Shell is launched after connecting possible devices, but
before the reset that is meant to automatically make them visible.

The new PcdUefiShellDefaultBootEnable must be set to TRUE to enable
this behaviour. The Pcd is set to false by default.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Tested-by: Patrik Berglund <patrik.berglund@arm.com>

ArmPkg/Library/PlatformBootManagerLib/PlatformBootManagerLib.inf

@@ -2,7 +2,7 @@
 #  Implementation for PlatformBootManagerLib library class interfaces.
 #
 #  Copyright (C) 2015-2016, Red Hat, Inc.
-#  Copyright (c) 2014, ARM Ltd. All rights reserved.<BR>
+#  Copyright (c) 2014 - 2023, Arm Ltd. All rights reserved.<BR>
 #  Copyright (c) 2007 - 2014, Intel Corporation. All rights reserved.<BR>
 #  Copyright (c) 2016, Linaro Ltd. All rights reserved.<BR>
 #
@@ -29,6 +29,7 @@
   PlatformBm.h
 
 [Packages]
+  ArmPkg/ArmPkg.dec
   EmbeddedPkg/EmbeddedPkg.dec
   MdeModulePkg/MdeModulePkg.dec
   MdePkg/MdePkg.dec
@@ -55,6 +56,7 @@
   gEfiMdePkgTokenSpaceGuid.PcdUgaConsumeSupport
 
 [FixedPcd]
+  gArmTokenSpaceGuid.PcdUefiShellDefaultBootEnable
   gEfiMdeModulePkgTokenSpaceGuid.PcdEmuVariableNvModeEnable
   gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString
   gEfiMdePkgTokenSpaceGuid.PcdUartDefaultBaudRate