cros_setup_toolchains: don't emerge an empty package list

When running this with only a single post_cross_item to update, |cmd|
ended up being ['/mnt/host/source/chromite/bin/parallel_emerge',
'--oneshot', '--update']. Avoid that.

BUG=chromium:1179917
TEST=`sudo cros_setup_toolchains --debug --nousepkg`

Change-Id: I5ec5508260bcd70372bd6be1abfb09f1610f0b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2860395
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 23e743c..b0910c2 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -557,12 +557,12 @@
     cmd.extend(packages)
     cros_build_lib.run(cmd)
   else:
-    post_cross_cmd = cmd[:]
-    cmd.extend([pkg for pkg in packages if pkg not in post_cross_pkgs])
-    cros_build_lib.run(cmd)
+    pre_cross_items = [pkg for pkg in packages if pkg not in post_cross_pkgs]
+    if pre_cross_items:
+      cros_build_lib.run(cmd + pre_cross_items)
     post_cross_items = [pkg for pkg in packages if pkg in post_cross_pkgs]
-    if len(post_cross_items) > 0:
-      cros_build_lib.run(post_cross_cmd + post_cross_items)
+    if post_cross_items:
+      cros_build_lib.run(cmd + post_cross_items)
   return True