The u-boot used by Arch Linux ARM is a base compile from hno. We don't even bother setting anything for the default environment storage, and let uEnv.txt or boot.scr sort it all out.
Here are the base defaults: https://github.com/hno/uboot-allwinner/ ... n4i.h#L176
When you want to override one of these, remember there is no "\0" that you need to add to the end of your lines with uEnv.txt!