rootfs.py: for dpkg/opkg, don't install postinsts if package management is present

rootfs.py: for dpkg/opkg, don't install postinsts if package management is present

If package management is present opkg/dpkg will bring the original
copy of the postinsts scripts with the metadata and will be able to
handle postinsts just fine. In fact, it is preferred to let package
management handle the postinsts scripts in this case since it will
keep the package managers database up-to-date too. The run-postinsts
scripts will make sure the package manager gets invoked instead of
the scripts directly.

Note: Before commit 5aae19959a44 ("rootfs.py: Change logic to
unistall packages") rootfs.py did not install /etc/$pm-postinsts
too. It is not clear whether that change was intentionally or just
a bug. This commit fixes/reverts that aspect of the commit.

(From OE-Core rev: 4b571c59e3e6bd8b22a63f547a95757aeba5b638)

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>