[lwIP] code cleanup for Kconfig.

1. code cleanup for Kconfig.
2. fix the compiling warning for BYTE_ORDER.

Author: BernardXiong

components/net/Kconfig

@@ -3,7 +3,7 @@ menu "Network stack"
 menu "light weight TCP/IP stack"
 config RT_USING_LWIP
     bool "Enable lwIP stack"
-	select RT_USING_DEVICE
+    select RT_USING_DEVICE
     default n
 
     if RT_USING_LWIP

components/net/lwip-1.4.1/src/lwipopts.h

@@ -36,7 +36,10 @@
 #define LWIP_HAVE_LOOPIF            0
 
 #define LWIP_PLATFORM_BYTESWAP      0
+
+#ifndef BYTE_ORDER
 #define BYTE_ORDER                  LITTLE_ENDIAN
+#endif
 
 /* #define RT_LWIP_DEBUG */