It isn't all that bad.
Buildroot will build u-boot, and even apply the changes if provided in the form of patches that it can handle.
Perhaps a quicker path is to learn how to use the "Network boot" feature already in the Kindle's u-boot build.
Then the machine would be running (in RAM) a kernel and system image of our choice.
A handy thing to have when trying to recover Kindles with nearly dead batteries.