Cleanup hton*/ntoh* function handling and platform abstraction

Let lwip use functions/macros prefixed by lwip_ internally to avoid naming clashes with external #includes.
Remove over-complicated #define handling in def.h
Make functions easier to override in cc.h. The following is sufficient now (no more LWIP_PLATFORM_BYTESWAP):
#define lwip_htons(x) <your_htons>
#define lwip_htonl(x) <your_htonl>

src/include/netif/ppp/eui64.h

@@ -84,7 +84,7 @@ typedef union
 
 #define eui64_set32(e, l)	do {			\
 				(e).e32[0] = 0;		\
-				(e).e32[1] = htonl(l);	\
+				(e).e32[1] = lwip_htonl(l);	\
 				} while (0)
 #define eui64_setlo32(e, l)	eui64_set32(e, l)
 

src/include/netif/ppp/mppe.h

@@ -59,7 +59,7 @@
  * This is not nice ... the alternative is a bitfield struct though.
  * And unfortunately, we cannot share the same bits for the option
  * names above since C and H are the same bit.  We could do a u_int32
- * but then we have to do a htonl() all the time and/or we still need
+ * but then we have to do a lwip_htonl() all the time and/or we still need
  * to know which octet is which.
  */
 #define MPPE_C_BIT		0x01	/* MPPC */