kedos is simple OS for raspberry pi 2.
Useful links:
https://github.com/dwelch67/raspberrypi : Many examples and articles.
https://github.com/jncronin/rpi-boot : Simple bootloader for raspberry pi with not bad file system implementation.
https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface : Description of mailbox interface.
http://wiki.osdev.org/Main_Page Must read for hobby's kernel developer.
https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/ Good tutorial for raspberry programming with assembler language.