device-purism-librem5: Run hciattach for Bluetooth (MR 5281)

This gets Bluetooth working for the Sparklan card.

btattach doesn't work on the Librem5 at the moment (see [1]) so even
though it's deprecated let's use hciattach to get things working.

1: https://source.puri.sm/Librem5/librem5-base/-/issues/76



Signed-off-by: Alistair Francis <alistair@alistair23.me>

[ci:skip-build] already built successfully in CI