u-boot needs extra config.txt section on Pi Zero 2

[Dave Jones]

On the Raspberry Pi Zero 2, under Ubuntu Focal, and extra [pi0] section pointing at the u-boot build for the Pi 3 (which is largely the same as the Zero 2) is required to permit the image to boot "out of the box" without manually removing u-boot first.

A postinst script for the u-boot-rpi binary package should check for the existence of this section and add it, but only if other u-boot referencing sections are found (to account for the scenario of removal of those sections, which is a fairly common practice by those wishing to boot from USB MSD).

[Dave Jones (waveform)]

Addendum: this is only required on Focal as Groovy images onwards switched away from using u-boot, and isn't required on Bionic as support for the Zero 2 only extends to the most recent LTS (focal).

[Dave Jones (waveform)]

Note to self: don't forget to update the pi-gadget too (section needs adding for core20 images)

[Dave Jones (waveform)]

Fixed in 20.04.5