Fix default argument in toolhelper.py

Allow the client to pass None instead of the empty array.

Change-Id: I89704c1891a7c69275c99b1469dbf88b460e830a

tools/toolhelper.py

diff --git a/tools/toolhelper.py b/tools/toolhelper.py
index cc1a9cb..1eee1f7 100644
--- a/tools/toolhelper.py
+++ b/tools/toolhelper.py
@@ -14,9 +14,10 @@
 def run(tool, args, build=None, debug=True,
         profile=False, track_memory_file=None, extra_args=None,
         stderr=None, stdout=None, return_stdout=False, timeout=0, quiet=False,
-        cmd_prefix=[]):
+        cmd_prefix=None):
   cmd = []
-  cmd.extend(cmd_prefix)
+  if cmd_prefix:
+    cmd.extend(cmd_prefix)
   if build is None:
     build, args = extract_build_from_args(args)
   if build: