snap-kernel-uc22 lacks atheros firmware

[Summary]
The Atheros firmware files aren't available in the snap-kernel-uc22 snap, used in Ubuntu Core Desktop, so the Bluetooth module in Asus Zenbook UX21-e (and probably in a lot more devices) doesn't work.

[Steps to reproduce]
Boot, from a power-down state, a device with an Atheros-based Bluetooth module, into Ubuntu Core Desktop, open a root terminal and type

    bluetoothctl devices

It returns the error 'No default controller available', because the Bluetooth
module firmware wasn't loaded.

Now, from a power-down state, boot into an Ubuntu live session (for example,
from an USB pendrive), and when the desktop appears, reboot the computer
(without powering it down) into Ubuntu Core Desktop and type again in a root
terminal

    bluetoothctl devices

This time there is no error, because the Bluetooth device firmware was already
loaded in the Ubuntu live session.

[Expected result]
The call to `bluetoothctl devices` should not return any error.

[Actual result]
The call to `bluetoothctl devices` returns the 'No default controller available'
error.

[Failure rate]
3/3

[Stage]
Patch available to fix it.