rootfs: add option to allow delayed postinsts on read-only rootfs
Example use case in OpenBMC: rootfs is squashfs and the system has either
overlayfs for whole rootfs or for some parts (e.g. /etc).
This option will allow to create migration one-shot postinsts using
"pkg_postinst_ontarget_${PN}" routines defined in recipes to fix
files under upper workdir in overlayfs.
Signed-off-by: Anton D. Kachalov <rnouse@google.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
