[src][irq] 添加 WEAK 修饰，中断嵌套数可由外部提供。

src/irq.c

@@ -105,7 +105,7 @@ RTM_EXPORT(rt_interrupt_leave);
  *
  * @return the number of nested interrupts.
  */
-rt_uint8_t rt_interrupt_get_nest(void)
+RT_WEAK rt_uint8_t rt_interrupt_get_nest(void)
 {
     rt_uint8_t ret;
     rt_base_t level;