opencv: Only enable 'libav' support if commercial license is whitelisted

The libav support requires the commercial license to be
whitelist. This makes opencv to work in non-commercial setups without
needing to change the PACKAGECONFIG settings manually.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>