Thanks for reaching out!
If you would like to improve Edison's boot time, I'd suggest you to modify the image to remove any packages not required by your project like it is discussed on Re: Cleaning up an Edison image .
However, if what you like to achieve is, as you mentioned, modify the time it takes U-boot to start, you will probably have to modify Edison's U-boot which is not supported.
Nevertheless, if you would like to try this, you can check Edison's U-boot source in GitHub - 01org/edison-u-boot .
I hope this is useful.
Remove dfu call from U-Boot for first. (Just from environment)
Compile as much as possible into kernel for second. (And do not compile what is not used)
You may try to use initrd instead of real rootfs (This will be RAM consuming, but you can really adjust what you have there. Mine is abou 25Mb unpacked)
That's what came to my mind.