main/linux-firmware: zst downstream Pi bluez/wifi firmware & optimize

Downstream Pi bluez/wifi firmware overwrite has been altered by initial move to compressed upstream firmware, leading to confusing duplication and uncompressed files for Pi.
Any raspberrypi-named firmware is now (again) from Pi Foundation repo, and compressed.
Upstream original files are kept (if not named against raspberrypi): any colliding downstream file is renamed with .rpiltd. to denote its origin.
Pi firmware files are no-longer duplicated and follow symlinks as appropriate to save storage.