lwip: align the pbuf on 32 resp. 64 bit addresses

Minor speed improvements of ~6Mbit. Additionally a ethernet frame fits now
into one memory allocation per pbuf. Beforehand two were allocated - one being
1514 bytes and another one being 2 bytes (monitored by instrumenting copy loop
in libports/src/lib/lwip/platform/nic.cc).

libports/include/lwip/lwipopts.h

@@ -54,6 +54,13 @@
 
 #define MEM_LIBC_MALLOC             1
 #define MEMP_MEM_MALLOC             1
+#ifdef __x86_64__
+#define MEM_ALIGNMENT               8
+#else
+#define MEM_ALIGNMENT               4
+#endif
+#define ETH_PAD_SIZE                2
+
 #define DEFAULT_ACCEPTMBOX_SIZE   128
 #define TCPIP_MBOX_SIZE           128