[QT5][QT6] Handle both QT versions dynamically

• add meta-qt6 to bbalyer.conf
• add BBMASK for meta-qtX layer if QT Y is requested
• correctly handle qtmultimedia for QT6 to only include ffmpeg when the license has been accepted.

Signed-off-by: Nicola Sparnacci nicola.sparnacci@seco.com