drivers: create BT_DRIVERS Kconfig entry

Fixes: #38403

Changing Bluetooth drivers from being a menu into a menuconfig.
This aligns the Bluetooth driver configuration with other driver
configurations as well as provides a setting which identifies if
Bluetooth drivers has been enable.

This further helps to avoid empty Zephyr libraries for bluetooth
samples.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>