All available Edison documentation is in Intel® Edison Documents and Guides . Currently there is not a document specific to u-boot. You can search for uboot in the community and find some information but not a specific document. I found this thread that I think you will find useful Re: trying to modify boot process with syslinux . We encourage you to create one and share it with the community!
If you decide to share it you can post it in a community thread not as a question but as a discussion, I’m sure this will help many users.