gst1-imx: fix v4l2 plugins disable options

Latest bump to v0.13.0 didn't take care of the following upstream
commit:

  7bfbf71 v4l2: Rename to v4l2video and add switches to enable/disable sink/source

To address that, this commit:

 - Uses the new name of the v4l2 source configure script option

 - Introduce a new Config.in option to control the build of the v4l2
   sink support

Fixes:

  http://autobuild.buildroot.net/results/5c404a2a28a4052056e5c9a7edda40641fe55f9e



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>