The prelink recipe is checking if image-prelink is in USER_CLASSES. This is the convention for enabling this, but it doesn't have to be enabled that way - it could even be inherited in an image recipe rather than at the configuration level and it would still work, but this check wouldn't get the correct answer. Given that this check is done in a postinstall and it's when it's on the host that we want to do the check, I wonder, would it be possible to export another variable during do_rootfs that would make it into the environment such that the script could check it?
http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=05a67104676e192e959150eb49902bed82681545