exposing serial number block.

sim/libmbit.ts

@@ -233,6 +233,13 @@ namespace pxsim.control {
             ? b.id.slice(0, 4) 
             : 'abcd';
     }
+    
+    export function deviceSerialNumber(): number {
+        let b = board();
+        return parseInt(b && b.id         
+            ? b.id.slice(1) 
+            : '42');
+    }
 
     export function onEvent(id: number, evid: number, handler: RefAction) {
         pxt.registerWithDal(id, evid, handler)

sim/state.ts

@@ -511,7 +511,7 @@ namespace pxsim {
 
         constructor() {
             super()
-            this.id = "b" + Math.random().toString().slice(1);
+            this.id = "b" +Math_.random(2147483647);
             this.animationQ = new AnimationQueue(runtime);
             this.bus = new EventBus(runtime);
             this.radio = new RadioBus(runtime);