microbit: changed the default target

Previously the default target in .yotta.json was targetting
a namespace that did exist originally, but no longer does.

The default target is now bbc-microbit-classic-armcc.

.yotta.json

@@ -1,5 +1,5 @@
 {
   "build": {
-    "target": "bbc-microbit-armcc,*"
+    "target": "bbc-microbit-classic-armcc,*"
   }
 }