Re: raspberry pi 4 arm64 test image

Back in the arm32 world, the contrib/miller/9/bcm kernel source
is now stable and useable on pi4, after fixing some old emmc bugs,
and thanks to cinap for spotting that no-execute bits have to be
set in device space page tables to prevent speculative instruction
fetches from device registers (!).

Wifi is still the only supported device so far.  Rather than
re-inventing wheels, I'll adopt 9front drivers for the rest
(after I've caught up with some other work).