Code to flash an ESP8266 from an arduino. See article on https://www.atrakeur.com/blog/art28-comment_charger_son_propre_firmware_sur_lesp8266_a_travers_un_arduino

FlashingEsp8266.cpp

#define RST_PIN  8  //Reset pin
#define CHPD_PIN 9  //CHPD pin
#define GPIO_PIN 10 //Flashmode pin

void setup() {
  //Open serial
  Serial.begin(115200);
  Serial1.begin(115200);
  
  //Set pin modes on arduino
  pinMode(RST_PIN, OUTPUT);
  pinMode(CHPD_PIN, OUTPUT);
  pinMode(GPIO_PIN, OUTPUT);
  
  //CHPD et RESET in running state (HIGH)
  //GPIO0 in flash mode (LOW)
  digitalWrite(CHPD_PIN, HIGH);
  digitalWrite(RST_PIN, HIGH);
  digitalWrite(GPIO_PIN, LOW);
  
  //Wait for serial to come up (Leonardo?)
  while(!Serial);
}

void loop() {
    //Upload firmware from computer to ESP
    if ( Serial.available() )       {  Serial1.write( Serial.read() );  }
    //Upload ESP responces to computer
    if ( Serial1.available() )   {  Serial.write( Serial1.read() );  }
}