updated pxt references + hero banner

libs/core/linux.cpp

@@ -271,6 +271,10 @@ void setupThread(Action a, TValue arg = 0, void (*runner)(Thread *) = NULL, TVal
     }
 }
 
+void releaseFiber() {
+    // called from main in pxt.cpp
+}
+
 void runInParallel(Action a) {
     setupThread(a);
 }