If you want an OS to handle edge cases like booting from the network (that can be arbitrarily complex), reusing an existing one makes a lot more sense than writing a custom one from scratch. Make a very basic bootloader that will init the minimal amount of hardware and then load a user-supplied boot image from the flash. That's it. Linux typically boots this way on non-embedded systems.
What operating system would you use, that is small enough to be burned into a flash chip on the motherboard, and capable of booting arbitrary target operating systems?
Linux, for example? 32MB is several times bigger than your average OpenWRT image. If you need more for the drivers or anything else then you already have more on your motherboard, and surely won't be limited by the OS overhead.
I said "and capable of booting arbitrary target operating systems" for a reason. Linux can kexec itself and I believe modded (but not mainline) FreeBSD. UEFI is rather more general.
