puzzles: fix a few things

- old acos() function was broken, replaced with a call to atan2(); this fixes "Cube!"
- Makefile extended to support building "unfinished games", but not enabled
- a backdrop issue fixed in rockbox.c

Change-Id: I9393e958d43de32f4ccf18e1cb409f75c2e1ed3c

apps/plugins/puzzles/rockbox.c

@@ -699,6 +699,10 @@ static void do_configure_item(config_item *cfg)
     {
         bool res = cfg->ival != 0;
         rb->set_bool(cfg->name, &res);
+
+        /* seems to reset backdrop */
+        rb->lcd_set_backdrop(NULL);
+
         cfg->ival = res;
         break;
     }