lwip update

* The mdns responder has been reworked to lower stack and memory usage. This is
  a variation on the upstream code, it use malloc whereas the upstream code uses
  pools. The high stack usage of the mdns responder was problem for
  esp-open-rtos, so we might have to maintain the differences for now.

* Improved lwip core locking, and lock checking. Upstream improvements, that
  need some added support from esp-open-rtos specific code. More core lock is
  performed when calling from the esp-open-rtos code now, so a little safer. The
  checking is not enforced, but projects might see warning messages and might
  want to look into them.

* The esp-open-rtos lwip support has been sync'ed with the new freertos port
  included with lwip. There are still some minor differences.

* A few lwip timer bugs have been resolved. This might help resolve some issues.

* Plus it picks up all the other upstream fixes and improvements.

* The default lwip stack has been lowered from 768 words to 480 words,
  due to the reduced stack usage by the mdns responder.

lwip/include/lwipopts.h

@@ -81,7 +81,9 @@
  * UNLOCK_TCPIP_CORE().
  * Your system should provide mutexes supporting priority inversion to use this.
  */
+#ifndef LWIP_TCPIP_CORE_LOCKING
 #define LWIP_TCPIP_CORE_LOCKING         1
+#endif
 
 /**
  * LWIP_TCPIP_CORE_LOCKING_INPUT: when LWIP_TCPIP_CORE_LOCKING is enabled,
@@ -95,6 +97,48 @@
 #define LWIP_TCPIP_CORE_LOCKING_INPUT   0
 #endif
 
+/**
+ * Macro/function to check whether lwIP's threading/locking
+ * requirements are satisfied during current function call.
+ * This macro usually calls a function that is implemented in the OS-dependent
+ * sys layer and performs the following checks:
+ * - Not in ISR
+ * - If @ref LWIP_TCPIP_CORE_LOCKING = 1: TCPIP core lock is held
+ * - If @ref LWIP_TCPIP_CORE_LOCKING = 0: function is called from TCPIP thread
+ * @see @ref multithreading
+ */
+#ifndef LWIP_ASSERT_CORE_LOCKED
+void sys_check_core_locking(void);
+#define LWIP_ASSERT_CORE_LOCKED()       sys_check_core_locking()
+#endif
+
+/**
+ * Called as first thing in the lwIP TCPIP thread. Can be used in conjunction
+ * with @ref LWIP_ASSERT_CORE_LOCKED to check core locking.
+ * @see @ref multithreading
+ */
+#ifndef LWIP_MARK_TCPIP_THREAD
+void sys_mark_tcpip_thread(void);
+#define LWIP_MARK_TCPIP_THREAD()        sys_mark_tcpip_thread()
+#endif
+
+#if LWIP_TCPIP_CORE_LOCKING
+
+#ifndef LOCK_TCPIP_CORE
+void sys_lock_tcpip_core(void);
+#define LOCK_TCPIP_CORE()          sys_lock_tcpip_core()
+#endif
+
+#ifndef UNLOCK_TCPIP_CORE
+void sys_unlock_tcpip_core(void);
+#define UNLOCK_TCPIP_CORE()        sys_unlock_tcpip_core()
+#endif
+
+#else
+#define LOCK_TCPIP_CORE()
+#define UNLOCK_TCPIP_CORE()
+#endif /* LWIP_TCPIP_CORE_LOCKING */
+
 /*
    ------------------------------------
    ---------- Memory options ----------
@@ -483,6 +527,21 @@
 #define LWIP_NETIF_HOSTNAME             1
 #endif
 
+/**
+ * LWIP_NETIF_API==1: Support netif api (in netifapi.c)
+ */
+#ifndef LWIP_NETIF_API
+#define LWIP_NETIF_API                  0
+#endif
+
+/**
+ * LWIP_NETIF_STATUS_CALLBACK==1: Support a callback function whenever an interface
+ * changes its up/down status (i.e., due to DHCP IP acquisition)
+ */
+#ifndef LWIP_NETIF_STATUS_CALLBACK
+#define LWIP_NETIF_STATUS_CALLBACK      0
+#endif
+
 /**
  * LWIP_NETIF_TX_SINGLE_PBUF: if this is set to 1, lwIP *tries* to put all data
  * to be sent into one single pbuf. This is for compatibility with DMA-enabled
@@ -515,7 +574,7 @@
  * sys_thread_new() when the thread is created.
  */
 #ifndef TCPIP_THREAD_STACKSIZE
-#define TCPIP_THREAD_STACKSIZE          768
+#define TCPIP_THREAD_STACKSIZE          480
 #endif
 
 /**
@@ -672,6 +731,20 @@
    ---------------------------------------
 */
 
+/*
+   ---------------------------------------
+   ---------- mDNS options ---------------
+   ---------------------------------------
+*/
+
+/**
+ * LWIP_MDNS_RESPONDER_QUEUE_ANNOUNCEMENTS==1: Unsolicited announcements are
+ * queued and run from a timer callback.
+ */
+#ifndef LWIP_MDNS_RESPONDER_QUEUE_ANNOUNCEMENTS
+#define LWIP_MDNS_RESPONDER_QUEUE_ANNOUNCEMENTS     1
+#endif
+
 /*
    ---------------------------------------
    ---------- Debugging options ----------
@@ -855,6 +928,11 @@
  */
 #define IP6_DEBUG                       LWIP_DBG_OFF
 
+/**
+ * MDNS_DEBUG: Enable debugging for multicast DNS.
+ */
+#define MDNS_DEBUG                      LWIP_DBG_OFF
+
 /*
    --------------------------------------------------
    ---------- Performance tracking options ----------