add void rt_hw_us_delay(rt_uint32_t us) for mini2440

bsp/mini2440/drivers/board.c

@@ -125,4 +125,10 @@ void rt_hw_board_init(void)
 #endif
 
 }
+
+void rt_hw_us_delay(rt_uint32_t us)
+{
+
+}
+
 /*@}*/