All postinstalls should be able to run at do_rootfs time.
http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=5d8a2e82e885b79c380ffaa5f613318f6d271e2c
Verified on master branch. Commit ID: 9ffee9add2d0380f4025e7a678a166f25a7d659e