Add a --platform flag

Projects may optionally specify their platform
(eg, groups="platform-linux" in the manifest).

By default, repo will automatically detect the platform. However,
users may specify --platform=[auto|all|linux|darwin].

Change-Id: Ie678851fb2fec5b0938aede01f16c53138a16537

command.py

@@ -15,6 +15,7 @@
 
 import os
 import optparse
+import platform
 import re
 import sys
 
@@ -69,7 +70,7 @@ class Command(object):
 
     groups = mp.config.GetString('manifest.groups')
     if groups is None:
-      groups = 'default'
+      groups = 'default,platform-' + platform.system().lower()
     groups = [x for x in re.split('[,\s]+', groups) if x]
 
     if not args: