parallel_emerge: call _emerge.main.clean_logs

Since we aren't using `emerge` or emerge_main(), the clean logs logic
doesn't get a chance to kick in.  Call the func directly ourselves so
that logs get cleaned.

BUG=chromium-os:28874
TEST=`./build_packages --board=daisy` cleaned out old logs for the daisy board

Change-Id: I674af21fbcdf9d50cecdf56e7ac4be694501878d
Reviewed-on: https://gerrit.chromium.org/gerrit/34703
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/parallel_emerge.py b/scripts/parallel_emerge.py
index 33a3223..3d15255 100644
--- a/scripts/parallel_emerge.py
+++ b/scripts/parallel_emerge.py
@@ -54,6 +54,7 @@
 from _emerge.actions import load_emerge_config
 from _emerge.create_depgraph_params import create_depgraph_params
 from _emerge.depgraph import backtrack_depgraph
+from _emerge.main import clean_logs
 from _emerge.main import emerge_main
 from _emerge.main import parse_opts
 from _emerge.Package import Package
@@ -1650,5 +1651,7 @@
     args += ["--exclude=sys-apps/portage"]
     os.execvp("sudo", args)
 
+  clean_logs(emerge.settings)
+
   print "Done"
   return 0