config: Make configs/common a Python package

Continue along the same line as the recent patch that made the
Ruby-related config scripts Python packages and make also the
configs/common directory a package.

All affected config scripts are updated (hopefully).

Note that this change makes it apparent that the current organisation
and naming of the config directory and its subdirectories is rather
chaotic. We mix scripts that are directly invoked with scripts that
merely contain convenience functions. While it is not addressed in
this patch we should follow up with a re-organisation of the
config structure, and renaming of some of the packages.

configs/dist/sw.py

@@ -37,10 +37,10 @@ from m5.defines import buildEnv
 from m5.objects import *
 from m5.util import addToPath, fatal
 
-addToPath('../common')
+addToPath('../')
 
-import Simulation
-import Options
+from common import Simulation
+from common import Options
 
 def build_switch(options):
     # instantiate an EtherSwitch