Please only use rsetup to update the system, as otherwise you might get an unbootable system (but not from U-Boot update).
We are maintaining our system as a rolling release distro. This is unavoidable when you keep having new products to support and new SDK to update to, so you never can stop at some stable version and say no more breaking changes from now on. With apt upgrade, it won’t remove packages for this operation, which would result in partial upgrade and broken system. This is why you cannot use this command.
u-boot-* packages can be installed freely. They don’t modify the installed bootloader when package is installed, since there is no good way to tell where the bootloader is actually installed. Like you said, there is also the potential to break the system, so they are designed to NOT modify the system when installed (even if say we can figure out where to install them). User must explicitly install them from rsetup.