b2/: make top-level hierarchy item an action, not just a rule

This allows the definition of optional global fields. For now, we (ab)use
this for FP (footprint).

b2/HIERARCHY

@@ -5,8 +5,9 @@
 <comp> = R<C<L<D;	// order doesn't really matter
 <diode> = D<SCH<Z<TVS;	// D < SCH may make sense, the rest doesn't
 
+{ FP=* }
 T=<comp> {
-	R:	{ R=#R TOL=%R FP=* };
+	R:	{ R=#R TOL=%R };
 	C:	{ C=#F TOL=%C V=#V };
 	L:	{ L=#H TOL=%L I=#A };
 	D:	{ C=#F }
@@ -15,4 +16,4 @@ T=<comp> {
 			Z:	{ V=#V I=#A };
 			*:	{ Vf=#V Vr=#V I=#A };
 		};
-}
+};