Creating IPv6 only network

### Basic Infos

- [X] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [X] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [X] I have tested that the issue is present in current master branch (aka latest git).
- [X] I have searched the issue tracker for a similar issue.
- [X] If there is a stack dump, I have decoded it.
- [X] I have filled out all fields below.

#### Platform

- Hardware: ESP8266
- Core Version: latest git
- Development Env: Arduino IDE
- Operating System: Ubuntu

### Settings in IDE

- Module: Nodemcu
- Flash Size: 4MB
- lwip Variant: v6 Lower Memory
- Reset Method: nodemcu
- Flash Frequency: [40Mhz]
- CPU Frequency: 80Mhz
- Upload Using: SERIAL
- Upload Speed: 115200(serial upload only)

### Problem Description

I want to create a complete ipv6 network. But I,m not getting how to disable the ipv4 addresses. Can you please provide the solution for this?
```
#include "lwip/opt.h"
#include "lwip/def.h"

#include "lwip/ip4_addr.h"
#include "lwip/ip6_addr.h"
#define LWIP_IPV4 0
#define LWIP_IPV6 1
#define LWIP_DHCP 0
const char *ssid ="ssid_name";
const char *password = "pass";

void setup() {
// put your setup code here, to run once:

Serial.begin(115200);
WiFi.begin(ssid,password);
while(WiFi.status() != WL_CONNECTED ){
delay(500);
Serial.println("Connecting to wifi");
}
Serial.println("Connected to Wifi");
for (bool configured = false; !configured;) {
for (auto addr : addrList)
if ((configured = ! addr.isLocal()
&&addr.isV6() // uncomment when IPv6 is mandatory
&& addr.ifnumber() == STATION_IF
)) {
break;
}
Serial.println('.');
delay(500);
}
for (auto a : addrList) {
Serial.printf("IF='%s' IPv6=%d local=%d hostname='%s' addr= %s\n",
a.ifname().c_str(),
a.isV6(),
a.isLocal(),
a.ifhostname(),
a.toString().c_str());
}
}

void loop() {
// put your main code here, to run repeatedly:

}
```