[BREAKING] Disable WiFi at boot by default (#7902)

* Disable WiFi at boot by default

* +define WIFI_IS_OFF_AT_BOOT

* remove now useless example

* mv enableWiFiAtBootTime() to core_esp8266_features.h

* sync with master

* per @earlephilhower review: a file was missing

* doc

* WiFi persistence is now false by default

* fix doc

* ditto

* doc: remove sphinx warnings (fix links and formatting)

* fix link name

* fix doc

* legacy: restore persistence

* undeprecate preinit()

* move force modem up to when mode has changed (per @mcspr review)

* do not wake up from sleep when mode if OFF

* fix doc per review

libraries/esp8266/examples/SerialStress/SerialStress.ino

@@ -57,12 +57,6 @@ void error(const char* what) {
   }
 }
 
-void preinit() {
-  // (no C++ in function)
-  // disable wifi
-  ESP8266WiFiClass::preinitWiFiOff();
-}
-
 void setup() {
   pinMode(LED_BUILTIN, OUTPUT);
 

libraries/esp8266/examples/UartDownload/UartDownload.ino

@@ -98,12 +98,6 @@ void proxyEspSync() {
 //
 ////////////////////////////////////////////////////////////////////////////////
 
-void preinit() {
-  // (no C++ in function)
-  // disable wifi
-  ESP8266WiFiClass::preinitWiFiOff();
-}
-
 void setup() {
   // For `proxyEspSync()` to work, the Serial.begin() speed needs to be
   // 115200bps. This is the data rate used by esptool.py. It expects the Boot