Refactor download API in the dev server.

Add comments and take named params.

BUG=chromium-os:24604
TEST=Ran dev server, verify download API works.

Change-Id: Ie7462e134dffe87f6e171e867ae62a752589a449
Reviewed-on: https://gerrit.chromium.org/gerrit/13593
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Ready: Frank Farzan <frankf@chromium.org>
Tested-by: Frank Farzan <frankf@chromium.org>
diff --git a/downloader.py b/downloader.py
index dc7a07f..a775250 100755
--- a/downloader.py
+++ b/downloader.py
@@ -47,7 +47,7 @@
         return 'Success'
 
       cherrypy.log('Downloading build from %s' % archive_url, 'DOWNLOAD')
-      staging_dir = tempfile.mkdtemp(suffix=archive_url.replace('/', '_'))
+      staging_dir = tempfile.mkdtemp(suffix='_'.join([board, build]))
       devserver_util.DownloadBuildFromGS(
           staging_dir=staging_dir, archive_url=archive_url, build=build)