[2.0.x] Add tone support for Due

[thinkyhead]

This is a quick-and-dirty implementation of tone supporting a single pin for the Due platform. This will allow you to play single tones on a Piezo speaker. It uses index 6 in the interrupt table.

[bwgeez02]

I'm getting a compile error.

Arduino: 1.9.0-beta (Windows 7), Board: "Arduino Due (Programming Port)"

sketch\src\HAL\HAL_DUE\usb\usb_task.c.o: In function `HAL_init':

sketch\src\HAL\HAL_DUE\usb/usb_task.c:259: undefined reference to `toneInit'

collect2.exe: error: ld returned 1 exit status

Multiple libraries were found for "LiquidCrystal.h"
 Used: C:\Users\Brian\Documents\Arduino\libraries\LiquidCrystal
 Not used: K:\arduino-1.9.0-beta-windows\arduino-PR-beta1.9-BUILD-37\libraries\LiquidCrystal
exit status 1
Error compiling for board Arduino Due (Programming Port).

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.
[Configuration.zip](https://github.com/MarlinFirmware/Marlin/files/1756592/Configuration.zip)
[Configuration_adv.zip](https://github.com/MarlinFirmware/Marlin/files/1756593/Configuration_adv.zip)

[thinkyhead]

Now patched, @bwgeez02

[bwgeez02]

Thanks! Sorry about the revert. I'll be more careful now.