Yocto boot2qt for the Raspberry Pi 4, both Qt 5.15 and 6.2

In this guide we'll build a linux distribution for Raspberry Pi 4, using the Yocto project and the `boot2qt` stack provided by Qt. This `boot2qt` image can be written to an SD card and when booted up, the Raspberry Pi runs a software stack that integrates nicely with Qt Creator (the Qt IDE), for example, one click deployment to the device. You can run your own Qt application on the Pi, full screen, it will boot right into it.
This guide covers both Qt 5.15 and Qt 6.2. The 5.15 build process is a bit convoluted due to a few upstream issues. The guide also covers changing the default startup app to your own app and Qt Creator integration.