A USB webcam app using libuvc

Behaviour
---------

The app requests an USB session for accessing the webcam and a GUI session in
order to render the camera into.

Configuration
-------------

! + config | width: 640 | height: 480 | format: yuv | fps: 30
!   + libc | stdout: /dev/log | stderr: /dev/log | rtc: /dev/rtc | pipe: /pipe
!   + vfs
!     + dir pipe | + pipe
!     + dir dev
!       + device | + log
!       + inline rtc | : 2018-01-01 00:01

The camera mode can be configured via the following optional config attributes.

:width/height: frame size
  (default is 640x480)

:format: currently 'yuv' (YUY2) and 'mjpeg' (MJPG) formats are supported
  (default is auto selection/detection)

:fps: frame interval in frames-per-second
  (default is auto selection/detection)

In case there is no suitable mode present, the app will dump the camera
supported formats during startup.
