[U-Boot] environment expectations of config_distro_bootcmd.h

Is there any documentation as to what environment variables a platform must provide if it uses config_distro_bootcmd.h. ${scriptaddr} seems clear but what about e.g. kernel_addr_r and friends? I suppose the use of the pxe commands has some implicit dependencies (pxe_addr_r?)

Along the same lines is there any documentation regarding which variables a boot.scr which expects to be called via this mechanism can rely on. e.g. devtype/devnum/bootpart/prefix etc. Also foo_addr_r again I suppose?

There is some info in the README but it says things like "these variables don't have to be defined for all boards" and worse "some boards use these variables for other purposes." ;-)

Thanks, Ian.