temp-netsender: build failure with ESP8266

When building for `ESP8266` I get the following:
```
sketch/NetSender.cpp: In function 'void NetSender::wifiOff()':
NetSender.cpp:679:13: error: 'WIFI_MODE_NULL' was not declared in this scope
WiFi.mode(WIFI_MODE_NULL);
^
exit status 1
'WIFI_MODE_NULL' was not declared in this scope
```
The relevant code is here
```

void wifiOff() {
#ifdef ESP8266
wifi_station_disconnect();
bool stopped = false;
for (int attempts = 0; !stopped && attempts < WIFI_ATTEMPTS; attempts++) {
stopped = (wifi_station_get_connect_status() == DHCP_STOPPED);
delay(WIFI_DELAY);
}
if (!stopped) {
log(logError, "DHCP not stopping.");
restart(bootWiFi, true);
}
wifi_set_opmode(NULL_MODE);
wifi_set_sleep_type(MODEM_SLEEP_T);
wifi_fpm_open();
wifi_fpm_do_sleep(0xFFFFFFF);
#endif
WiFi.mode(WIFI_MODE_NULL);
delay(WIFI_DELAY);
#ifdef ESP32
#endif
log(logDebug, "WiFi off");
}
```

`nonarduino.h` contains this macro, but `nonarduino.h` is only included for `__linux__`.

Start in NetSender.cpp at wifiOff() and compare the ESP8266 build path with the nonarduino.h macro definition. Build the project for ESP8266 to reproduce the missing WIFI_MODE_NULL declaration; done means that target builds successfully without breaking the other guarded paths.

Written by the indexing model from the issue text.