It's sort of out of the scope of Buildroot, but there are different systems like mender or swupdate that cover updates. They tend to need some level of platform specific changes because embedded systems these days have 7 levels of bootloaders.
I’ve been using fwup (https://github.com/fwup-home/fwup) in embedded Linux consumer products for almost 10 years and it has Just Worked.
Supports A/B updates, uboot integration, and signed update files.
Not sure about buildroot, but in Yocto it’s fairly straightforward to write an image creation recipe that bundles your ext4 filesystem image(s) into an update file you can deploy.
