[U-Boot] hardware versions to Kernel

I have an ARM CPU on a board that has 3 optional sub boards plugged into it, all of which have 3 hardware rev bits connected to the CPU

Rather than getting the CPU to do mmap type stuff or sys/class/gpio stuff to read those versions in the kernel, is there a way for u-boot to push that information ( even it's a U32 or a string ) into the kernel somewhere that can be parsed? I tried some code in the board startup to create env vars to append into the bootargs but they aren't evaluated each boot properly ( boards are optional and revs can change ) so cat /proc/cmdlne stayed the same value all the time

Is there a better way to do this?

thanks

Andy