Previous __FlashStringHelper implementation was defines as a char which brought problem in case the method with char* parameter used overloading with __FlashStringHelper* parameter (they was identical). Now __FlashStringHelper is defined as a class and all casts between char* and __FlashStringHelper* are made with reinterpret_cast sugar.
example:
```cpp
//done once on WiFi init
configTime(-7200, 3600, "pool.ntp.org");
//get local time
struct tm timeinfo;
if(!getLocalTime(&timeinfo)){
Serial.println("Failed to obtain time");
return;
}
//print time
Serial.println(&timeinfo);
//print time with different format
Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");
```
fixes: https://github.com/espressif/arduino-esp32/issues/29
