buildstats-summary/toaster: Cope with removal of get_bn()

The buildstats changes removed the no longer needed get_bn() function,
replace this with references to BUILDNAME.

(From OE-Core rev: e1a37899da56014693f08d1c39cb6ec0a4ed2bf4)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/buildstats-summary.bbclass

@@ -3,8 +3,7 @@ python buildstats_summary () {
     import collections
     import os.path
 
-    bn = get_bn(e)
-    bsdir = os.path.join(e.data.getVar('BUILDSTATS_BASE', True), bn)
+    bsdir = e.data.expand("${BUILDSTATS_BASE}/${BUILDNAME}")
     if not os.path.exists(bsdir):
         return