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>

https://forum.pjrc.com/threads/23681-Many-axis-joystick?p=41943&viewfull=1#post41943

This patch simply checks if NUM_ENDPOINTS is defined, and will bypass the
built-in USB descriptor, device and memory code, and allows a custom setting.
This bug does not affect the IDE unless someone adds the custom option to
the boards.txt file, and provides replacement code in a library or sketch.
It can also be used to turn off USB by providing no replacements, while
allowing speeds >= 20MHz.