Bluetooth: Move hci_uart UART Kconfig to the top-level file

To allow for hci_uart builds that do not include the controller code,
move the UART Kconfig option used by the sample up one level so that it
is shared by all configurations using Bluetooth:

Jira: ZEP-2132

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>