Modify to work with cros_workon.

This fixes bug 5686 where the stable version of the package gets
installed when the 9999 is unmasked.

BUG=5868
TEST=Verified that 9999 version of the package now gets installed.

Verified the following sequence:
1. verified stable gets installed
2. start power_manager
3. verified 9999 gets installed
4. stop power_manager
5. verified that stable gets installed

Change-Id: I26d600692e0e359d6ac153cdbca70ac5ae5d420f

Review URL: http://codereview.chromium.org/3125017
diff --git a/devserver.py b/devserver.py
index 3737cc0..73aa3ba 100644
--- a/devserver.py
+++ b/devserver.py
@@ -37,6 +37,10 @@
     err = os.system(emerge_command)
     if err != 0:
       raise Exception('failed to execute %s' % emerge_command)
+    eclean_command = 'eclean-%s -d packages' % input.board
+    err = os.system(eclean_command)
+    if err != 0:
+      raise Exception('failed to execute %s' % emerge_command)
 
 def OverrideWSGIServer(server_address, wsgi_app):
   """Creates a CherryPyWSGIServer instance.