mbed-os/LwIP changes and fixes in auto-IP for Bonjour Conformance Test.

Changes:
  1. Following issues are fixed in LwIP for AutoIP.
     a) Fixed bug in max conflict rate limitting.
        - According to RFC section RFC 3927 Section 2.2.1 conflict probe interval
          should be increased to 60 seconds, once conflict count reaches after
          MAX_CONFLICTS (i.e., 10) counts.
        - The initial value of 'autoip->tried_llipaddr' is 0. Hence the probe
          interval (i.e., autoip->ttw) should be increased to 60 secs
          when 'autoip->tried_llipaddr >= MAX_CONFLICTS'

     b) Added code to free 'autoip' client in autoip_stop() API.
        - New 'autoip' client is allocated in autoip_start() API, and the client
          is not freed during autoip_stop(). This would result in memory leak
          if not freed.
        - Updated autoip_stop() API to take care of releasing the memory allocated
          for 'autoip' client.
  2. Introduced a configurable macro "MBED_CONF_LWIP_DHCP_TIMEOUT" in "lwipopts.h"
     to configure DHCP timeout based on the usecase requirement. For example:
     bonjour conformance test would need a DHCP timeout value which is grater than
     320 secs to run mDNS probing test to verify protocol compilance of the implementation.

features/lwipstack/lwip/src/core/ipv4/lwip_autoip.c

@@ -94,6 +94,9 @@
 static err_t autoip_arp_announce(struct netif *netif);
 static void autoip_start_probing(struct netif *netif);
 
+/* static variables */
+static u8_t is_autoip_alloced = 0;      /* Set when 'struct autoip' is allocated dynamically and assigned to 'netif' */
+
 /**
  * @ingroup autoip
  * Set a statically allocated struct autoip to work with.
@@ -278,6 +281,7 @@ autoip_start(struct netif *netif)
                   ("autoip_start(): could not allocate autoip\n"));
       return ERR_MEM;
     }
+    is_autoip_alloced = 1;
     /* store this AutoIP client in the netif */
     netif_set_client_data(netif, LWIP_NETIF_CLIENT_DATA_INDEX_AUTOIP, autoip);
     LWIP_DEBUGF(AUTOIP_DEBUG | LWIP_DBG_TRACE, ("autoip_start(): allocated autoip"));
@@ -318,7 +322,7 @@ autoip_start_probing(struct netif *netif)
    * acquiring and probing address
    * compliant to RFC 3927 Section 2.2.1
    */
-  if (autoip->tried_llipaddr > MAX_CONFLICTS) {
+  if (autoip->tried_llipaddr >= MAX_CONFLICTS) {
     autoip->ttw = RATE_LIMIT_INTERVAL * AUTOIP_TICKS_PER_SECOND;
   }
 }
@@ -353,6 +357,16 @@ autoip_stop(struct netif *netif)
   LWIP_ASSERT_CORE_LOCKED();
   if (autoip != NULL) {
     autoip->state = AUTOIP_STATE_OFF;
+    /* If autoip is dynamically allocated in start, free autoip structure and reset autoip index in netif */
+    if (is_autoip_alloced) {
+      /* Reset the auto IP index and then free autoip structure */
+      netif_set_client_data(netif, LWIP_NETIF_CLIENT_DATA_INDEX_AUTOIP, NULL);
+      mem_free(autoip);
+      autoip = NULL;
+      is_autoip_alloced = 0;
+    } else {
+      autoip->tried_llipaddr = 0;
+    }
     if (ip4_addr_islinklocal(netif_ip4_addr(netif))) {
       netif_set_addr(netif, IP4_ADDR_ANY4, IP4_ADDR_ANY4, IP4_ADDR_ANY4);
     }

features/lwipstack/lwipopts.h

@@ -57,7 +57,13 @@
 #define BOTH_ADDR_TIMEOUT           0
 #endif
 
+// Configurable DHCP timeout. DHCP timeout can be configured for specific usecase requirement.
+#ifdef MBED_CONF_LWIP_DHCP_TIMEOUT
+#define DHCP_TIMEOUT                (MBED_CONF_LWIP_DHCP_TIMEOUT)
+#else
 #define DHCP_TIMEOUT                60
+#endif
+
 #define LINK_TIMEOUT                60
 
 #define PREF_IPV4                   1