commands: PushImages, log --buildroot.

PushImages invokes the pushimage script as a function call, not an
external script, and wasn't logging all command line options that
would have been passed to it.

BUG=b:117812180
TEST=run_tests

Change-Id: I07f3254dea9f2d58a41f66b55f200fc7d06d28b2
Reviewed-on: https://chromium-review.googlesource.com/1292353
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Alec Thilenius <athilenius@google.com>
diff --git a/cbuildbot/commands.py b/cbuildbot/commands.py
index 93f19bb..cae7929 100644
--- a/cbuildbot/commands.py
+++ b/cbuildbot/commands.py
@@ -2271,6 +2271,9 @@
   if sign_types:
     log_cmd.append('--sign-types=%s' % ' '.join(sign_types))
 
+  if buildroot:
+    log_cmd.append('--buildroot=%s' % buildroot)
+
   log_cmd.append(archive_url)
   logging.info('Running: %s', cros_build_lib.CmdToStr(log_cmd))