add reference to Skiboot, add it to graphic as a LinuxBoot pre-stage

[orangecms]

and also https://youtu.be/4JbDb4bRBK4 to the book as related talks

[pamolloy]

As I recall https://github.com/BayLibre/k-boot boots from a mix of ATF/BL2 and BL32/OP-TEE. Might as well add them to the diagram as well.