v4.2.0

• CHANGED: lsats instruction used for saturation in the mixer
• CHANGED: Mixer task communication scheme simplified, aiding code reuse &
  performance
• CHANGED: Audio Class Control Interface no longer presented in descriptors
  if NUM_USB_CHAN_IN and NUM_USB_CHAN_OUT are both zero
• CHANGED: Buffering sub-system no longer spawns if NUM_USB_CHAN_IN and
  NUM_USB_CHAN_OUT are both zero
• CHANGED: Communication of commands between tasks now uniformly uses
  control tokens. Potentially making mix & match of components more tractable
  in the future
• Changes to dependencies:
  • lib_spdif: 6.1.0 -> 6.1.1
  • lib_xud: 2.3.1 -> 2.3.2