barebox

The project began as U-Boot-v2 at the German firm Pengutronix, a rewrite of U-Boot, whose codebase had grown for years without much in the way of architecture. The differences became large enough that it went its own way, and in 2009 it was renamed barebox. The ideas are borrowed from the Linux kernel: a proper driver model, devicetree, mountable filesystems, and a shell with real scripts instead of U-Boot's environment variables stuffed with embedded commands.

barebox has found its footing mainly in industrial ARM systems, often in NXP's i.MX world, where Pengutronix is active. U-Boot still supports more boards and has the larger ecosystem. So the choice between them is rarely technical — it comes down to whatever the board vendor's BSP happened to ship with.