more lwIP physical interfaces (#6680)

This commit adds W5500 W5100 and ENC28j60 drivers from @njh with credits
They are available in libraries/
An example is added in W5500 examples directory

plus:
* Extract dhcp server from lwip2 and add it to the core as a class.
  It must always be present, it is linked and can be called by fw on boot.
  So it cannot be stored in a library.
* ethernet: static or dhcp works
* PPPServer: example
* bring WiFi.config() to the lwIP generic interface (argument reorder common function)
* move hostname() from WiFI-STA to generic interface
* remove non readable characters from dhcp-server comments
* dhcp-server: magic_cookie is part of bootp rfc
* fixes from d-a-v/W5500lwIP#17
* enable lwip_hook_dhcp_parse_option()
* +ethernet tcp client example in w5500 library examples

Author: d-a-v

cores/esp8266/LwipDhcpServer-NonOS.cpp

@@ -0,0 +1,64 @@
+/*
+    lwIPDhcpServer-NonOS.cpp - DHCP server wrapper
+
+    Copyright (c) 2020 esp8266 arduino. All rights reserved.
+    This file is part of the esp8266 core for Arduino environment.
+
+    This library is free software; you can redistribute it and/or
+    modify it under the terms of the GNU Lesser General Public
+    License as published by the Free Software Foundation; either
+    version 2.1 of the License, or (at your option) any later version.
+
+    This library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+    Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public
+    License along with this library; if not, write to the Free Software
+    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+// STARTS/STOPS DHCP SERVER ON WIFI AP INTERFACE
+// these functions must exists as-is with "C" interface,
+// nonos-sdk calls them at boot time and later
+
+#include <lwip/init.h> // LWIP_VERSION
+
+#include <lwip/netif.h>
+#include "LwipDhcpServer.h"
+
+extern netif netif_git[2];
+
+// global DHCP instance for softAP interface
+DhcpServer dhcpSoftAP(&netif_git[SOFTAP_IF]);
+
+extern "C"
+{
+
+    void dhcps_start(struct ip_info *info, netif* apnetif)
+    {
+        // apnetif is esp interface, replaced by lwip2's
+        // netif_git[SOFTAP_IF] interface in constructor
+        (void)apnetif;
+
+#if 0
+        // can't use C++ now, global ctors are not initialized yet
+        dhcpSoftAP.begin(info);
+#else
+        (void)info;
+        // initial version: emulate nonos-sdk in DhcpServer class before
+        //                  trying to change legacy behavor
+        // `fw_has_started_softap_dhcps` will be read in DhcpServer::DhcpServer
+        // which is called when c++ ctors are initialized, specifically
+        // dhcpSoftAP intialized with AP interface number above.
+        fw_has_started_softap_dhcps = 1;
+#endif
+    }
+
+    void dhcps_stop()
+    {
+        dhcpSoftAP.end();
+    }
+
+} // extern "C"