优化修改ymodem代码, 添加注释

ymodem/ymodem.c

@@ -48,7 +48,7 @@ static unsigned long str_to_u32(char *str)
 typedef struct
 {
   void *h; RingBuf rx_buf;
-  RingBufElement rx_buf_storage[2*(PACKET_1K_SIZE+PACKET_OVERHEAD)];
+  RingBufElement rx_buf_storage[2*(PACKET_1K_SIZE+PACKET_OVERHEAD)]; // 最大缓存2包数据
 } SYmodemUart;
 
 static SYmodemUart s_myUart;