lwip: Added support for configurable LwIP hooks

Added lwIP hooks which could be optionally overwritten in the
application code. These three options are provided in Kconfig:
* NONE: No hook support
* DEFAULT: Default implementation is provided. If IDF doesn't
have a specific hook implementation, an empty stub is provided, which
could be overwritten by strong implementation in application code.
* CUSTOM: Hooks are declared only to be implemented in application code.

Merges https://github.com/espressif/esp-idf/pull/6034

components/lwip/port/esp32/hooks/lwip_default_hooks.c

@@ -0,0 +1,40 @@
+// Copyright 2020 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "lwip_default_hooks.h"
+
+#define __weak __attribute__((weak))
+
+#ifdef CONFIG_LWIP_HOOK_IP6_ROUTE_DEFAULT
+struct netif *__weak
+lwip_hook_ip6_route(const ip6_addr_t *src, const ip6_addr_t *dest)
+{
+    LWIP_UNUSED_ARG(src);
+    LWIP_UNUSED_ARG(dest);
+
+    return NULL;
+}
+#endif
+
+#ifdef CONFIG_LWIP_HOOK_NETCONN_EXT_RESOLVE_DEFAULT
+int __weak lwip_hook_netconn_external_resolve(const char *name, ip_addr_t *addr, u8_t addrtype, err_t *err)
+{
+    LWIP_UNUSED_ARG(name);
+    LWIP_UNUSED_ARG(addr);
+    LWIP_UNUSED_ARG(addrtype);
+    LWIP_UNUSED_ARG(err);
+
+    return 0;
+}
+#endif