I would like to find a way to run sed scripts on files within a Yocto-generated OS from a .bbappend file. My OS has a read-only rootfs, which seems to stop any possibility of a post-installation script. Specifically, I need to make these changes to `/etc/default/ssh` (as run after booting the generated OS):

```
sed -i 's/var\/run/etc/' /etc/default/ssh 
sed -i 's/_readonly//' /etc/default/ssh

```

Here's my `openssh_7.1p1.bbappend` which I've created in an attempt to solve these problems:

```
FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
SRC_URI += " \
    file://ssh_host_dsa_key.pub  \
    file://ssh_host_rsa_key.pub  \
    ...
    "

do_install_append() {
    sed -i 's/var\/run/etc/' ${D}${sysconfdir}/default/ssh
    sed -i 's/_readonly//' ${D}${sysconfdir}/default/ssh

    # these lines work fine
    install -m 0755 ${WORKDIR}/ssh_host_dsa_key          ${D}/etc/ssh
    install -m 0755 ${WORKDIR}/ssh_host_dsa_key.pub      ${D}/etc/ssh
    ...
}
FILES_${PN} += "${sysconfdir}/default/ssh"

#these lines work
FILES_${PN} += "${sysconfdir}/ssh/ssh_host_dsa_key"
FILES_${PN} += "${sysconfdir}/ssh/ssh_host_dsa_key.pub"
...

```

BitBake fails during the execution of `do_install_append()` with this error: 

```
sed: can't read ${TMPDIR}/work/x86-poky-linux/openssh/image/etc/default/ssh: No such file or directory

```

(where TMPDIR is my actual tmp directory) Obviously this file doesn't exist because the proper copy is created in a separate `MULTIMACH_TARGET_SYS` directory (i.e. not `x86-poky-linux`) by image.bbclass. 

Is this sort of thing possible to do from within a .bbappend file (or some other compartmentalized way)? I have found a way to do it within a .inc file with `ROOTFS_POSTPROCESS_COMMAND` within my core-image, but this method is leading to a poor organizational structure.