pi: reprovision all Pi's with Raspbian Buster, add docs

[rvagg]

Raspbian Stretch to Raspbian Buster. I also discovered that a sound driver update sometime this year was the cause of boot instability, so that's now disabled at boot and they're running much smoother now.

Have updated the additional docs in here for Pi's to be current and make the setup (somewhat) reproducable.

arm-fanned is back online now after its hiatus, still ironing out some minor hiccups though.

Ref: #1840
Ref: nodejs/node#30786

[rvagg]

I also think that arm-fanned wasn't properly handling Node <12. I enabled git in the parent job so it could get the NODEJS_MAJOR_VERSION to perform its subjob branching. That may have been coming from node-test-commit already but it was only checking for NODE_MAJOR_VERSION which I don't think is anywhere. Regardless, it meant that you couldn't run arm-fanned independently and get the appropriate version running. So now node-test-binary-arm and node-test-binary-arm-12+ are selected accordingly.
When doing that, I also noticed that node-test-binary-arm was selecting wheezy on the Pi2's which was removed some time ago, so now it's only wheezy on the Pi1's and stretch on the rest which means wheezy can be properly retired when Node 8 goes away I think.

[mhdawson]

Ruberstampish LGTM