Make sure to call SDK functions to start and stop DHCP server

As noticed in esp8266#8582 (comment)
Can't really use `server.begin()` and `server.end()` directly, only
default static IP is applied to the interface since DHCP server is
deemed 'running' (see `wifi_softap_dhcps_status()` return value)

Author: mcspr

libraries/ESP8266WiFi/src/ESP8266WiFiAP.cpp

@@ -167,8 +167,7 @@ bool ESP8266WiFiAPClass::softAP(const char* ssid, const char* psk, int channel,
         DEBUG_WIFI("[AP] softap config unchanged\n");
     }
 
-    auto& server = softAPDhcpServer();
-    server.end();
+    wifi_softap_dhcps_stop();
 
     // check IP config
     struct ip_info ip;
@@ -179,17 +178,13 @@ bool ESP8266WiFiAPClass::softAP(const char* ssid, const char* psk, int channel,
                 IPAddress(192, 168, 4, 1),
                 IPAddress(192, 168, 4, 1),
                 IPAddress(255, 255, 255, 0));
-            if(!ret) {
-                DEBUG_WIFI("[AP] softAPConfig failed!\n");
-                ret = false;
-            }
         }
     } else {
         DEBUG_WIFI("[AP] wifi_get_ip_info failed!\n");
         ret = false;
     }
 
-    server.begin();
+    wifi_softap_dhcps_start();
 
     return ret;
 }
@@ -227,9 +222,10 @@ bool ESP8266WiFiAPClass::softAPConfig(IPAddress local_ip, IPAddress gateway, IPA
     info.gw.addr = gateway.v4();
     info.netmask.addr = subnet.v4();
 
-    auto& server = softAPDhcpServer();
-    server.end();
-
+    // use SDK function for dhcps, not just server.begin()
+    // setting info with static IPs will fail otherwise
+    // (TODO: dhcps_flag seems to store 'SDK' DHCPs status)
+    wifi_softap_dhcps_stop();
     if(!wifi_set_ip_info(SOFTAP_IF, &info)) {
         DEBUG_WIFI("[APConfig] wifi_set_ip_info failed!\n");
         ret = false;
@@ -246,14 +242,17 @@ bool ESP8266WiFiAPClass::softAPConfig(IPAddress local_ip, IPAddress gateway, IPA
     dhcp_lease.end_ip.addr = ip.v4();
     DEBUG_WIFI("[APConfig] DHCP IP end: %s\n", ip.toString().c_str());
 
+    auto& server = softAPDhcpServer();
     if(!server.set_dhcps_lease(&dhcp_lease))
     {
-        DEBUG_WIFI("[APConfig] wifi_set_ip_info failed!\n");
+        DEBUG_WIFI("[APConfig] server set_dhcps_lease failed!\n");
         ret = false;
     }
 
-    server.setRouter(true); // send ROUTER option with netif's gateway IP
-    server.begin();
+    // send ROUTER option with netif's gateway IP
+    server.setRouter(true);
+
+    wifi_softap_dhcps_start();
 
     // check config
     if(wifi_get_ip_info(SOFTAP_IF, &info)) {