All postinstalls should be able to run at do_rootfs time.
Filed a bug on fontconfig bugzilla asking for the best way to implement this and if the changes we would make to fontconfig would be accepted upstream. https://bugs.freedesktop.org/show_bug.cgi?id=59456
patches merged: http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=c31bf6514bdda951b315f16aa640f9d46232c060 http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=49f5daf9b71a4bd9c839b348c9819019903a5f2e http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=f33014f60722efd9ca7dd03aeee3dd46d8035616 http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=e46f33688c011e5e087267a18437316c05b22f14 http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=be82ae98ef7a8525a850a1724426a5ff51932775
Verified on master branch. Commit ID: 9ffee9add2d0380f4025e7a678a166f25a7d659e