Return true for boolean test on Serial

Fixes: espressif#270

Author: me-no-dev

cores/esp32/HardwareSerial.cpp

@@ -89,5 +89,5 @@ size_t HardwareSerial::write(const uint8_t *buffer, size_t size)
 
 HardwareSerial::operator bool() const
 {
- return _uart != 0;
+ return true;
 }