pi4 firmware update breaks 9pi, and how to fix it

pi4 firmware update breaks 9pi, and how to fix it

[Richard Miller]

If you're thinking of updating the firmware (fixup*.dat, start*.elf)
on a raspberry pi 4, for example to experiment with the (beta) network
booting support, watch out!  A series of commits by the Pi Foundation
on https://github.com/raspberrypi/firmware in September and October
managed to mess up 9pi in various ways.  The latest firmware (as of
11 December) is better, except that the gic (interrupt controller) is
no longer enabled by default for non-devicetree operating systems.
(This has to be done in secure mode before the OS starts.)

To counteract this, add the parameter 'enable_gic=1' to the pi4
section in config.txt.

I think this won't affect 9front, which uses devicetree, as long
as you update the .dtb files when updating firmware.