Box led3: Increase stack requirement

A stack overflow can be observed when using the mbed OS with RTX2. This
is the minimum value that allows the app to work again.

Author: AlessandroA

source/led3.cpp

@@ -19,7 +19,7 @@ static void led3_main(const void *);
  * stack size can be smaller as we do not do anything special in them. */
 UVISOR_BOX_NAMESPACE(NULL);
 UVISOR_BOX_HEAPSIZE(3 * 1024);
-UVISOR_BOX_MAIN(led3_main, osPriorityNormal, 1024);
+UVISOR_BOX_MAIN(led3_main, osPriorityNormal, 2048);
 UVISOR_BOX_CONFIG(box_led3, acl, 512, box_context);
 
 #define uvisor_ctx ((box_context *) __uvisor_ctx)