fix module compile error when RTT_ROOT variable not to be set

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1738 bbd45198-f89e-11dd-88c7-29a3b14d5316

examples/module/SConstruct

@@ -6,7 +6,7 @@ import rtconfig
 if os.getenv('RTT_ROOT'):
     RTT_ROOT = os.getenv('RTT_ROOT')
 else:
-    RTT_ROOT = os.path.normpath(os.getcwd() + '/../../..')
+    RTT_ROOT = os.path.normpath(os.getcwd() + '/../..')
 
 sys.path = sys.path + [os.path.join(RTT_ROOT, 'tools')]
 from building import *