Bluetooth: Audio: Remove hardcoded location and context for LC3 macros

The BT_CODEC_LC3_CONFIG_* macros had a hardcoded location
and context, which makes them a lot less usable.

Updates the macro, and the macros that used them.
This also removes the BT_CODEC_LC3_CONFIG and instead
just uses the BT_CODEC_LC3_CONFIG_N macro, which has
then been renamed to BT_CODEC_LC3_CONFIG.

As an addition, the macros and their input has
also been better documented.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>