Website: https://www.denx.de/wiki/U-Boot
U-Boot ("Das U-Boot") is a boot loader for embedded systems. It could theoretically be used for your desktop machine as well, but it is mainly intended to be used in embedded systems. U-Boot is configured very similarly to the Linux kernel: there is a .config file which can be generated and edited with an ncurses-based tool.
There's not a whole lot to say here about U-Boot. For the most part, you'll be modifying the source for your chip to add your own board's definitions and such to it, assuming the defaults for that don't already work. This is heavily dependent on what you need to do with your system and what hardware you have chosen, so I can't say much about that here.
U-Boot can boot from the network, various types of flash, hard drives, and so on. Just about any type of device is supported, or can be supported if you add the code for it. It can also be configured to use a serial console, and includes a bunch of handy command-line tools in a POSIX sh-like shell. These tools include (but are not limited to):
mwfor writing memory of various sizes at arbitrary addressesmdfor dumping memory at arbitrary addresses- commands for booting the system manually
Effectively, you can manually manipulate the majority of your system from the U-Boot shell. This allows you to test hardware without a device tree, kernel, or anything getting in your way--you can manually modify whichever registers you need.
There isn't much more to say about U-Boot here, as you will be doing a lot that is extremely specific to your use case. Look for docs on that, or roll your own.