Expose optional DHCP timout parameters to Ethernet:begin()

A no brainer update.
Expose optional timeout parameter values of dhcp for user to adjust on
their code.
Currently if dhcp server is not available, code will block inside
DhcpClass::request_DHCP_lease() for 60s. Having this parameters exposed
user can specify their timeout requirements without changing the lib
code.
Change is backward compatible as parameters are optional and defaults to
existing values.

Author: chaveiro

libraries/Ethernet/src/Ethernet.cpp

@@ -8,7 +8,7 @@ uint8_t EthernetClass::_state[MAX_SOCK_NUM] = {
 uint16_t EthernetClass::_server_port[MAX_SOCK_NUM] = { 
   0, 0, 0, 0 };
 
-int EthernetClass::begin(uint8_t *mac_address)
+int EthernetClass::begin(uint8_t *mac_address, unsigned long timeout, unsigned long responseTimeout)
 {
   static DhcpClass s_dhcp;
   _dhcp = &s_dhcp;
@@ -22,7 +22,7 @@ int EthernetClass::begin(uint8_t *mac_address)
   SPI.endTransaction();
 
   // Now try to get our config info from a DHCP server
-  int ret = _dhcp->beginWithDHCP(mac_address);
+  int ret = _dhcp->beginWithDHCP(mac_address, timeout, responseTimeout);
   if(ret == 1)
   {
     // We've successfully found a DHCP server and got our configuration info, so set things

libraries/Ethernet/src/Ethernet.h

@@ -20,7 +20,7 @@ class EthernetClass {
   // Initialise the Ethernet shield to use the provided MAC address and gain the rest of the
   // configuration through DHCP.
   // Returns 0 if the DHCP configuration failed, and 1 if it succeeded
-  int begin(uint8_t *mac_address);
+  int begin(uint8_t *mac_address, unsigned long timeout = 60000, unsigned long responseTimeout = 4000);
   void begin(uint8_t *mac_address, IPAddress local_ip);
   void begin(uint8_t *mac_address, IPAddress local_ip, IPAddress dns_server);
   void begin(uint8_t *mac_address, IPAddress local_ip, IPAddress dns_server, IPAddress gateway);