[yocto] Remove/prevent a package's installed files if another package present

[colin.helliwell at ln-systems.com]

I'd like to do a mod to my ppp recipe such that ip-up & ip-down aren't
installed if another package is in the image recipe i.e. set in
CORE_IMAGE_EXTRA_INSTALL.

I guess I could do a do_install_append() or a pkg_postinst_(), but is there
a way to detect the other package within the recipe? E.g. along the lines of
what PACKAGE_CONF can do for configuration.

Oh, and it needs to work after an update (e.g. rpm/deb) - don't want the
files (re)installed by the update. (So I don't believe
ROOTFS_POSTPROCESS_COMMAND is an option)