NET: Add a few missing configuration options to the Kconfig files

net/arp/Kconfig

@@ -20,6 +20,13 @@ config NET_ARPTAB_SIZE
 	---help---
 		The size of the ARP table (in entries).
 
+config NET_ARP_MAXAGE
+	int "Max ARP entry age"
+	default 120
+	---help---
+		The maximum age of ARP table entries measured in deciseconds.  The
+		default value of 120 corresponds to 20 minutes (BSD default).
+
 config NET_ARP_IPIN
 	bool "ARP address harvesting"
 	default n

net/arp/arp_table.c

@@ -125,7 +125,9 @@ void arp_timer(void)
   for (i = 0; i < CONFIG_NET_ARPTAB_SIZE; ++i)
     {
       tabptr = &g_arptable[i];
-      if (tabptr->at_ipaddr != 0 && g_arptime - tabptr->at_time >= UIP_ARP_MAXAGE)
+
+      if (tabptr->at_ipaddr != 0 &&
+          g_arptime - tabptr->at_time >= CONFIG_NET_ARP_MAXAGE)
         {
           tabptr->at_ipaddr = 0;
         }