- get rid of ridden V4L2_COMMON symbol

  This symbol is not needed anymore; it can be folded with V4L2
  one, simplifying the Kconfig a little bit;

- Comment why some Kconfig items are needed;

- Remove if test for MEDIA_CAMERA_SUPPORT, replacing it by
  depends on.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>