arm64/rpi4 support

[JedMeister]

arm64/rpi4 support

This is #14 - rebased on current master - https://github.com/turnkeylinux/fab/commit/998de3b2800843fd29e20334d2e1df5c3431cae6

It was previously merged into #16 - but has been separated again as it's arm64/rpi specific.

The non arm64/rpi commits from #16 are now in #26