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">
!   <vfs>
!     <dir name="dev"> <log/> <inline name="rtc">2018-01-01 00:01</inline> </dir>
!     <dir name="pipe"> <pipe/> </dir>
!  </vfs>
!  <libc stdout="/dev/log" stderr="/dev/log" rtc="/dev/rtc" pipe="/pipe"/>
! </conifg>

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.
