Add cros build subcommand and tests.

cros build is the command used to merge Portage packages to the chroot
or to a board.

Update specified package and all dependencies:
  cros build --board=lumpy power_manager
  cros build --host cros-devutils

Just build a single package:
  cros build --board=lumpy --no-deps power_manager

BUG=chromium-os:35483
TEST=Run unit tests and sample use cases.

Change-Id: I06cf80f13930356c2a98d45028c6f0ea64da1b88
Reviewed-on: https://gerrit.chromium.org/gerrit/39009
Commit-Ready: David James <davidjames@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/scripts/cros.py b/scripts/cros.py
index 532897d..05e289e 100644
--- a/scripts/cros.py
+++ b/scripts/cros.py
@@ -14,7 +14,10 @@
 
   subparsers = parser.add_subparsers(title='cros commands')
   for cmd_name, class_def in sorted(my_commands.iteritems(), key=lambda x:x[0]):
-    sub_parser = subparsers.add_parser(cmd_name, help=class_def.__doc__)
+    epilog = getattr(class_def, 'EPILOG', None)
+    sub_parser = subparsers.add_parser(
+        cmd_name, help=class_def.__doc__, epilog=epilog,
+        formatter_class=commandline.argparse.RawDescriptionHelpFormatter)
     class_def.AddParser(sub_parser)
 
   return parser