McuSupport: Store macros as part of McuSdkRepository

To support extending the macros and calling the
McuTargetFactory::expandVariables function statically, the macros are
added as part of the SdkRepository.

This commit also adds helper functions to extend macros other than the
ones created from the packages.

Change-Id: Ie7d2a9ad626782eec18738bdd3472ffd202e7a36
Reviewed-by: Rainer Keller <Rainer.Keller@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/mcusupport/mcusupportsdk.cpp

@@ -591,7 +591,11 @@ McuSdkRepository targetsFromDescriptions(const QList<McuTargetDescription> &desc
             mcuPackages.insert(package);
         }
     }
-    return McuSdkRepository{mcuTargets, mcuPackages};
+
+    McuSdkRepository repo{mcuTargets, mcuPackages};
+    repo.expandVariables();
+
+    return repo;
 }
 
 FilePath kitsPath(const FilePath &qtMcuSdkPath)