Add support for a dual serial port configuration (USB_DUAL_SERIAL),
providing a composite USB device, comprised of two serial ports.
The second serial port is called usb_serial2 (C) or SerialA (C++).

Note that no dummy C++ class is created if USB_DISABLED is defined,
unlike for the first port.

This increases binary size by ca. 1.2 KiB (720 bytes for USB buffers).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

Add preprocessor macros to control the naming of functions, structures
and classes, to ease the creation of multiple USB serial instances.
Name generation for each instance is controlled using the
USB_SERIAL_SUFFIX and SERIAL_CLASS_SUFFIX defines.

Move the preprocessor macro wrappers around the usb_serial_port data
members to usb_serial.h, as preprocessor macro names cannot be
parameterized.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

Extract core USB serial functionality in a new usb_serial_port C class,
to allow creating multiple USB serial instances without duplicating all
code.

The original API is retained by creating static inline functions and
preprocessor macros using the old names, which are wrappers around the
new usb_serial_port functions and data members.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

- usb_serial_class::readBytes() calls millis(), hence it should
    include "core_pins.h",
  - usb_serial_flush_callback() is a public function, but is not
    declared in the header file.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

https://forum.pjrc.com/threads/44904-Teensyduino-1-37-Beta-3?p=146286&viewfull=1#post146286

When clear function was added, it was not put in the null operations list when no USB enabled

On Teensy 3.x - we have Serial1.clear, Serial2.clear...

But not Serial.clear.

Serial.clear() exists for Teensy2, so thought I would try to add it for
Teensy 3s...

Sometimes the PC or Mac drivers are not actually ready to begin receiving data immediately after they cause DTR or RTS to assert.  This delay allows the PC to become ready before we tell sketches the Serial device is true (ready).

https://groups.google.com/a/arduino.cc/d/msg/developers/ls3hkviFYM4/XWT2LbfzNcgJ