Fix enabling v7M MPU too early

See issue ARMmbed#380.

Move call to vmpu_mpu_lock() after vmpu_load_box(). This guarantees that all
RBAR/RASR registers have been written-to (by vmpu_mpu_invalidate()) before the
MPU is enabled.

core/vmpu/src/armv7m/vmpu_armv7m.c

@@ -401,8 +401,6 @@ void vmpu_arch_init(void)
     /* Initialize static MPU regions. */
     vmpu_arch_init_hw();
 
-    vmpu_mpu_lock();
-
     /* Dump MPU configuration in debug mode. */
 #ifndef NDEBUG
     debug_mpu_config();

core/vmpu/src/vmpu.c

@@ -517,6 +517,9 @@ void vmpu_init_post(void)
 
     /* load boxes */
     vmpu_load_boxes();
+
+    /* enable mpu */
+    vmpu_mpu_lock();
 }
 
 static int copy_box_namespace(const char *src, char *dst)