Have host-tools populate build/host with 1) toybox, 2) all the remaining busybox commands the build can use, 3) symlinks for the other dozen or so commands the build needs. Then set $PATH to _just_ build/host, yanking the rest of the $PATH so it won't call other things from the host. This way, the environmental dependencies are more explicit. (Note that some commands symlinked from the host could be built by busybox, but the busybox versions of those don't work correctly in the build.)