Enable ARP processing for IP packets

uip/uip_arp.h

@@ -78,8 +78,8 @@ void uip_arp_init(void) __banked;
    inserts a new mapping if none exists. The function assumes that an
    IP packet with an Ethernet header is present in the uip_buf buffer
    and that the length of the packet is in the uip_len variable. */
-/*void uip_arp_ipin(void);*/
-#define uip_arp_ipin()
+void uip_arp_ipin(void) __banked;
+// #define uip_arp_ipin()
 
 /* The uip_arp_arpin() should be called when an ARP packet is received
    by the Ethernet driver. This function also assumes that the