cros_generate_update_payload: Allow gsbucket images

If 'path' type is passed to argparse arguments, then python prefixes the
path with the cwd and it messes up the path. By not assigning --image
and --src_image arguments a 'path' type, we can now send paths directory
to the gsbucket if needed for debugging purposes instead of first
maually downloading the images and passing them to
cros_generate_update_payload. This is basically how the paygen itself
works.

BUG=none
TEST=cros_generate_update_payload --image gs://foo_tgt --src_image
gs:/foo_src --output payload

Change-Id: Id70c1680ae796a7f01e08650856d86e46307e3f4
Reviewed-on: https://chromium-review.googlesource.com/1510132
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/scripts/cros_generate_update_payload.py b/scripts/cros_generate_update_payload.py
index 3200f89..aea1284 100644
--- a/scripts/cros_generate_update_payload.py
+++ b/scripts/cros_generate_update_payload.py
@@ -20,11 +20,13 @@
 def ParseArguments(argv):
   """Returns a namespace for the CLI arguments."""
   parser = commandline.ArgumentParser(description=__doc__)
-  parser.add_argument('--image', type='path',
-                      help='The image that should be sent to clients.')
-  parser.add_argument('--src_image', type='path',
-                      help='A source image. If specified, this makes a delta '
-                           'update.')
+  parser.add_argument('--image',
+                      help='The path (to local disk or Google Storage Bucket)'
+                      ' of the target image to build the payload for.')
+  parser.add_argument('--src_image',
+                      help='The path (to local disk or Google Storage Bucket)'
+                      ' of the source image. If specified, this makes a delta'
+                      ' update payload.')
   parser.add_argument('--output', type='path', help='Output file.')
   parser.add_argument('--private_key', type='path',
                       help='Path to private key in .pem format.')