package/kodi: Rework audio encoder support

Make option BR2_PACKAGE_KODI_OPTICALDRIVE invisible, it is only
needed when an audio encoder addon is enabled.

Add Kconfig submenu for audio encoders to prepare for addition of
more binary addons.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>