factory_install: Skip download if source URL is empty

When complete.gz is not served on the server, its source URL is not set.
In this case, installer will connect to default URL http://<host
ip>:<port>/update, but this URL is not used in Umpire server.
This CL fixes this issue such that if the source URL of a channel is not
set, it should be skipped.

BUG=chrome-os-partner:25791
TEST=download image from Umpire without complete scripte.

Change-Id: Iaa2fc535b7403b651816f78a0709c70dfdaa0c61
Reviewed-on: https://chromium-review.googlesource.com/202904
Reviewed-by: Rong Chang <rongchang@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Commit-Queue: Cheng-Yi Chiang <cychiang@chromium.org>
1 file changed