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

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).