bitbake: toaster: runbuilds Write the pidfile in python rather than shell script

Write the pid file out in the start up of this management command. This
ensures this has happened instead of relying on the shell command having
been run which may or may not be the case. This also makes it simpler for
testing.

Couple of clean ups of runbuilds as identified by pyflake

(Bitbake rev: 999e980ee1a58d16f33ef6c0e41aecdcd0206f39)

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/toaster

@@ -254,7 +254,7 @@ case $CMD in
             return 4
         fi
         export BITBAKE_UI='toasterui'
-        $MANAGE runbuilds & echo $! >${BUILDDIR}/.runbuilds.pid
+        $MANAGE runbuilds &
         # set fail safe stop system on terminal exit
         trap stop_system SIGHUP
         echo "Successful ${CMD}."