Bluetooth samples: Add USE_SPECIFIC_BROADCAST_CHANNEL Kconfig

Added USE_SPECIFIC_BROADCAST_CHANNEL which enable/disables the use of a
specific channel audio location for the broadcast audio sink sample.
If USE_SPECIFIC_BROADCAST_CHANNEL is set to no then first valid channel
is chosen. In this case broadcast channel with no audio location is
also supported.

Signed-off-by: Jens Rehhoff Thomsen <jthm@demant.com>