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>

Create a macro to emit all values for a CDC Interface Association
Descriptor, and use it.

This will avoid duplication when adding support for multiple USB serial
interfaces.

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

Create a macro to emit all values for the various CDC descriptors, and
use it.  Note that this does not include the CDC Interface Association
Descriptor, as the latter is needed only for composite USB devices.

This will avoid duplication when adding support for multiple USB serial
interfaces.

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

fixes #293, fixes #414

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

https://forum.pjrc.com/threads/34808-K66-Beta-Test?p=116690&viewfull=1#post116690

Results in valid serial # presented to USB host when T_3.6 is in HSRUN mode.

Add support for controlling left/right volume separately. Fix
wTotalLength after adding volume feature.

Change descriptor and code to handle features for USB Audio

https://forum.pjrc.com/threads/33017-Teensy-3-1-Mouse-horizontal-scroll

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.

http://forum.pjrc.com/threads/25482-Duplicate-usb-modem-number-HELP!