.pytool: Support FDT library.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4392
Add FDT support in EDK2 by submodule 3rd party libfdt
(https://github.com/devicetree-org/pylibfdt/tree/main/libfdt)

Add RequiredSubmodule object for CI setting.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Benny Lin <benny.lin@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

.pytool/CISettings.py

@@ -195,6 +195,8 @@ class Settings(CiBuildSettingsManager, UpdateSettingsManager, SetupSettingsManag
             "RedfishPkg/Library/JsonLib/jansson", False))
         rs.append(RequiredSubmodule(
             "UnitTestFrameworkPkg/Library/SubhookLib/subhook", False))
+        rs.append(RequiredSubmodule(
+            "MdePkg/Library/BaseFdtLib/libfdt", False))
         return rs
 
     def GetName(self):