Send DHCP request on ::begin even if lease exists (earlephilhower#2977)

When a link comes up, we were only sending a DHCP request if the existing
netif's ipaddress was 0.  When a DHCP lease is acquired at first that IP
is changed to the given address, and if we do another ::begin we wouldn't
dhcp_start to send a new request and use the old one (until its lease
expired).

In the case of network changing (i.e. WiFiMulti on different nets or
moving an Ethernet cable to another router) that old address would not
be valid.

Track if an IP address has been manually set and use that to determine
if DHCP needs to be re-requested instead of looking at the old netif's
ipaddress.

Fixes earlephilhower#2974

Author: earlephilhower

libraries/lwIP_Ethernet/src/LwipIntfDev.h

@@ -186,6 +186,7 @@ class LwipIntfDev: public LwipIntf, public RawDev {
     SPIClass& _spiUnit;
     SPISettings _spiSettings = SPISettings(4000000, MSBFIRST, SPI_MODE0);
     netif _netif;
+    bool _isDHCP = true;
 
     uint16_t _mtu;
     int8_t   _intrPin;
@@ -282,6 +283,8 @@ bool LwipIntfDev<RawDev>::config(const IPAddress& localIP, const IPAddress& gate
         return false;
     }
 
+    _isDHCP = (localIP.v4() == 0);
+
     IPAddress realGateway, realNetmask, realDns1;
     if (!ipAddressReorder(localIP, gateway, netmask, dns1, realGateway, realNetmask, realDns1)) {
         return false;
@@ -389,7 +392,7 @@ bool LwipIntfDev<RawDev>::begin(const uint8_t* macAddress, const uint16_t mtu) {
         _phID = __addEthernetPacketHandler([this] { this->handlePackets(); });
     }
 
-    if (localIP().v4() == 0) {
+    if (_isDHCP) {
         // IP not set, starting DHCP
         _netif.flags |= NETIF_FLAG_UP;
         switch (dhcp_start(&_netif)) {