Fix wrongly exported internal pin to the north side in H-Tree.

* Bug: In cumulus/plugins.block.HTree._rrouteHTree(), on the top level
    H-Tree, draw a wire to the north side of the abutment box *only*
    if the root (usually clock) signal is *external*. It may be that
    it is *internal*, so must be routed as a normal one to the root of
    the tree.

cumulus/src/plugins/alpha/block/htree.py

@@ -185,7 +185,7 @@ class HTree ( object ):
             self._connectLeaf( qt.br, ckNet, rightContact, brContact, rightX, blY )
         if qt.isRoot():
             ckNet = self.treeNet
-            if not self.spares.conf.isCoreBlock:
+            if not self.spares.conf.isCoreBlock and ckNet.isExternal():
                 trace( 550, '\tRemoving any previous pin...\n' )
                 pins = []
                 for pin in ckNet.getPins(): pins.append( pin )