eMMC boot partitions, it's better to call them "areas" to avoid confusion, are good candidates for uboot or other FW storage. would be if Rockchip ROM code did understand them. but it probably doesn't. at least rk3399 manual doesn't mention them. would be interesting to experiment. with Rockpro. they were intended for such usage, they free general purpose area for user and they provide redundancy (since they always come in two identical instances).
ANT - my hobby OS for x86 and ARM.