From J. Arrizza Wiki
These pages show how to set up uboot on the Raspberry Pi, no Linux. This is useful for example to run your own RTOS or "super-loop" application.
Another possibility is: https://github.com/brianwiddas/pi-baremetal but note that uboot has a lot of built-in facilities that are very useful during development.
- rPI U-boot - How to set up and run u-boot with no Linux
- rPI U-boot run s-rec application - How to load and run the s-rec hello_world application
- rPI U-boot Example App - How to add a your own example s-rec application to U-boot
- rPI U-boot Custom command - How to add a custom command to u-boot
- rPI U-boot run sd card application - How to load and run an application from an SD card
- rPI U-boot run TFTP application - How to load and run an application over tftp