rpm-postinsts: don't create broken postinst script

Not only was the variable reference in this line broken, but it wasn't
going to work anyway - we install the script directly into /etc/rcS.d
and not into /etc/init.d, so the code in update-rc.d.bbclass couldn't
find anything there. This resulted in a postinstall script for
rpm-postinsts being created in /etc/rpm-postinsts which can't work when
the root filesystem is read-only. To simplify things just remove the use
of update-rc.d.bbclass since we don't really need the added complexity
here.

Fixes [YOCTO #4222].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
rpm-postinsts: avoid errors during boot with read-only-rootfs enabled

* If /etc/rpm-postinsts doesn't exist, don't error
* If deleting the script errors, don't bother printing it (this will
  always happen if the root filesystem is read-only)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
initscripts: fix read-only-rootfs-hook.sh to avoid using unionfs

Unionfs isn't available everywhere, and we can get similar results (if
not quite as neatly) by using bind mounts + tmpfs and copying the data
over.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
initscripts: fix read-only-rootfs-hook.sh to start earlier

Mount /var/volatile ourselves so that we can set up the writable area
first. This fixes the urandom service not starting properly when
read-only-rootfs is enabled.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>