Closed source bootloader with very restricted license.
http://www.cnx-software.com/2016/07/19/raspberry-pi-bootloader-license-preludes-it-to-run-on-competing-broadcom-bcm283x-boards/
http://www.cnx-software.com/2016/07/19/raspberry-pi-bootloader-license-preludes-it-to-run-on-competing-broadcom-bcm283x-boards/