opencv: Add 'libv4l' PACKAGECONFIG

The i.MX SoCs need to allow the removal of libv4l support so a new
PACKAGECONFIG option has been included.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Gary Thomas <gary@mlbassoc.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>