[Tools] Fix the abspath issue of Rtt_Root in building.py.

tools/building.py

@@ -123,7 +123,7 @@ def PrepareBuilding(env, root_directory, has_libcpu=False, remove_components = [
     global Rtt_Root
 
     Env = env
-    Rtt_Root = root_directory
+    Rtt_Root = os.path.abspath(root_directory)
 
     # add compability with Keil MDK 4.6 which changes the directory of armcc.exe
     if rtconfig.PLATFORM == 'armcc':