adding buildBranch label to "N-Builds" so directory labels are correct in 'checkout'

releng.control/config.xml

@@ -292,6 +292,10 @@
                 <property
                     name="buildType"
                     value="N" />
+                <!-- in this context, buildBranch is simply a label -->
+                <property
+                    name="buildBranch"
+                    value="" />
                 <property
                     name="mapVersionTag"
                     value="HEAD" />