dhcp: add option for lowering discover interval

See espressif/ESP8266_RTOS_SDK@d64f04b

glue-lwip/arduino/lwipopts.h

@@ -968,6 +968,13 @@
 #if !defined LWIP_DHCP_MAX_DNS_SERVERS || defined __DOXYGEN__
 #define LWIP_DHCP_MAX_DNS_SERVERS       DNS_MAX_SERVERS
 #endif
+
+/**
+ * DHCP discover retransmission interval (milliseconds)
+ */
+#if !defined LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL || defined __DOXYGEN__
+#define LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL 250 // 250 - 1000
+#endif
 /**
  * @}
  */

glue-lwip/open/lwipopts.h

@@ -968,6 +968,13 @@
 #if !defined LWIP_DHCP_MAX_DNS_SERVERS || defined __DOXYGEN__
 #define LWIP_DHCP_MAX_DNS_SERVERS       DNS_MAX_SERVERS
 #endif
+
+/**
+ * DHCP discover retransmission interval (milliseconds)
+ */
+#if !defined LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL || defined __DOXYGEN__
+#define LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL 250 // 250 - 1000
+#endif
 /**
  * @}
  */

patches/dhcp-lower-retry-backoff.patch

@@ -0,0 +1,20 @@
+diff --git a/src/core/ipv4/dhcp.c b/src/core/ipv4/dhcp.c
+index 534574fe..88c7669f 100644
+--- a/src/core/ipv4/dhcp.c
++++ b/src/core/ipv4/dhcp.c
+@@ -1024,7 +1024,15 @@ dhcp_discover(struct netif *netif)
+     autoip_start(netif);
+   }
+ #endif /* LWIP_DHCP_AUTOIP_COOP */
++#ifdef LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL
++/**
++ * Since for embedded devices it's not that hard to miss a discover packet, so lower
++ * the discover retry backoff time from (2,4,8,16,32,60,60)s to (500m,1,2,4,8,15,15)s.
++ */
++  msecs = (u16_t)((dhcp->tries < 6 ? 1 << dhcp->tries : 60) * LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL);
++#else
+   msecs = (u16_t)((dhcp->tries < 6 ? 1 << dhcp->tries : 60) * 1000);
++#endif /* LWIP_DHCP_DISCOVER_RETRANSMISSION_INTERVAL */
+   dhcp->request_timeout = (u16_t)((msecs + DHCP_FINE_TIMER_MSECS - 1) / DHCP_FINE_TIMER_MSECS);
+   LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("dhcp_discover(): set request timeout %"U16_F" msecs\n", msecs));
+   return result;