Allow other ESP debug port class types

[bryceschober]

My use case is that I have implemented an alternative DEBUG_ESP_PORT object inheriting from the Print class, so that I can embed the debug output in our own framing method by overriding the lowest-level write() functions. The pointer comparison added in #4467 broke the ability to provide a different object type. Casting to void pointers before the comparison restores that ability.

My use case basically looks like:

/** This class has an interface like Arduino's HardwareSerial
 *  Its intent is to wrap plain-ascii debug messages in framed packets appropriate for our application.
 *
 *  This also works well with the ESP8266 Arduino debug output methodology that depends on a
 *  command-line definition of which HardwareSerial instance its source libraries should print to.
 *  By building with "-DDEBUG_ESP_PORT=Debug" (see public global instance below), the debug output of the source
 *  libraries can be cleanly wrapped as we desire in our application.
 */
class SerialDebugOut: public Print {
    // My private members for our framing format
public:
    SerialDebugOut();
    ~SerialDebugOut();
    // Implement a look-alike of HardwareSerial's method
    void begin(unsigned long baud);
    // Implement a look-alike of HardwareSerial's method
    void setDebugOutput(bool en);
    // Implement the virtual method from the Print interface for single-character writes
    size_t write( uint8_t byte );
    // Implement the virtual method from the Print interface for character-buffer writes
    size_t write(const uint8_t *buffer, size_t size);
};
// A single global definition must be provided by the user application
extern SerialDebugOut Debug;

I'll admit it's not as ideal as making the HardwareSerial class more inheritable, but it does function well.