[lwIP] code cleanup for Kconfig.
1. code cleanup for Kconfig.
2. fix the compiling warning for BYTE_ORDER.
#define LWIP_HAVE_LOOPIF 0
#define LWIP_PLATFORM_BYTESWAP 0
+
+#ifndef BYTE_ORDER
#define BYTE_ORDER LITTLE_ENDIAN
+#endif
/* #define RT_LWIP_DEBUG */
