[beluga] Specify exclusively `le` mode for Bluetooth

[beroset]

As reported by user @dv1 on Matrix chat, the default /etc/bluetooth/main.conf file is empty which means that the Bluetooth controller will default to dual mode rather than le mode. If this can be confirmed, it's an error. The minimal file should be:

[General]
ControllerMode = le

[MagneFire]

This is intentional, see the following for more context: https://github.com/AsteroidOS/meta-smartwatch/commit/85e529c615f87f4cd8c923116b6d37e0059b2f12

[dv1]

In discussions over on Matrix, @beroset noticed that you have to use bluetoothctl, and inside it, use the advertise on command. beluga is discoverable then even in le mode.

[MagneFire]

Hmm, seems logical. But this then begs the question why this is only required for hoki and beluga. Other watches do not seem to be affected by this.