[tools] Fixed precedence issue in cumulative attribute set-arithmetic

Basically this:
a | b - c  !=  (a | b) - c

Author: geky

tools/config.py

@@ -169,7 +169,8 @@ def strict_cumulative_overrides(self, overrides):
 
     def update_target(self, target):
         setattr(target, self.name, list(
-                set(getattr(target, self.name, [])) | self.additions - self.removals))
+                (set(getattr(target, self.name, [])) | self.additions) - self.removals))
+
 
 
 # 'Config' implements the mbed configuration mechanism