Bluetooth: add LED trigger for indicating HCI is powered up

Add support for LED triggers to the Bluetooth subsystem and add kernel
config symbol BT_LEDS for it.

For now one trigger for indicating "HCI is powered up" is supported.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>