The code for T3.x was broken, especially for USB2...

And the code was not setup for T4.x.  So added it.

Code is setup that if there are no eventResponders that have registered to do interrupt calls, don't have the systic code in place to look for it.

Also yield code.  Now figures out if the user has any code that installs their own serialEventX methods, to call them, but if they have left the default weak pointer versions, to not call them in yield.  Likewise will only look for eventRespnder events if the sketch has actually attached any events.  If none of these conditions are true, yield will just check one flag and return.

T3.x Serial events cleanup

Add support for a triple serial port configuration (USB_TRIPLE_SERIAL),
providing a composite USB device, comprised of three serial ports.
The third serial port is called usb_serial3 (C) or SerialB (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. 0.5 KiB (despite needing 720 more
bytes for USB buffers, as gcc-5.4.1 no longer decides to unroll a loop
over all endpoints in usb_isr()).

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

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>