cros: Fix `cros --help`

cros --help currently print:
usage: cros [--log-level {fatal,critical,error,warning,notice,info,debug}]
            ...
cros: error: too few arguments
And return code is 1, since it doesn't know the --help argument.

Also the output is at stderr and is not the same format that bash
completion is expecting.

Change add_help to always use the default value (which is True) fixes
this.

BUG=chromium:1029239
TEST=manually,
     `cros --help`, `cros flash --help`, `cros flash usb:// <image>` works.
TEST=./scripts/run_tests ./scripts/cros_unittest

Change-Id: I52de81443191af2944436cdb4c948ae205bd945f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1940036
Commit-Queue: Pi-Hsun Shih <pihsun@chromium.org>
Tested-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros.py b/scripts/cros.py
index 8368b29..cadba28 100644
--- a/scripts/cros.py
+++ b/scripts/cros.py
@@ -30,10 +30,7 @@
   Returns:
     A commandline.ArgumentParser object.
   """
-  # We need to omit help for the base parser so that we're able to parse out the
-  # subcommand for further parsing.
-  parser = commandline.ArgumentParser(caching=True, default_log_level='notice',
-                                      add_help=cmd_name is not None)
+  parser = commandline.ArgumentParser(caching=True, default_log_level='notice')
 
   subparsers = parser.add_subparsers(title='Subcommands', dest='subcommand')
   subparsers.required = True