Alpine's firmware packages have been updated to include the latest
raspberry pi firmware. This commit adjusts our aports.
Details:
* remove obsolete `aports/temp/linux-firmware`
* `firmware/firmware-pi-bluetooth`: rename to `device/bluetooth-raspberry-pi`
and remove firmware files (they are part of Alpine's `linux-firmware` now)
* `device-raspberry-pi`: depend on `bluetooth-raspberry-pi`
* `device-raspberry-pi`: note that Alpine's kernels depend non free firmware
* `device-raspberry-pi`: remove non free firmware subpackage
