The postinstall returns 1 when running on host which postpones it for first boot. This does not allow using RO rootfs.
Proposing for 1.5, it's quite intrusive for 1.4.1 (needs a native binary to be built).
Patch sent on the ML
Fix merged: http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=ec89446cb41c2be6aa76904f972d21611df90a5f