Install Gentoo on Radxa ROCK 5B+ on bcachefs rootfs

Recently I've been playing around with the Radxa ROCK 5B+. Turns out installing Gentoo on it is a rather involved process, especially if you want a bcachefs rootfs. For those of you wondering why on earth someone would want to do that: this post is not about the why, but about the how. I have my reasons. The ROCK 5B+ supports loading U-Boot from its SPI NOR flash chip. However, that chip is empty…