rpm: add PACKAGECONFIG for the systemd inhibit plugin

RPM ships a systemd inhibit plugin, that will tell systemd to inhibit a
reboot or sleep during a package upgrades.

For native RPM this is entirely useless, and for target it's only useful
if you're using systemd+logind+rpm.

This plugin uses DBus which means it depends on expat -> cmake-native ->
libarchive-native curl-native, which is quite a dependency tree to need
in early build (required to build packages via rpm-native).

It was previously forcibly disabled for native packages but the build
dependency on DBus remained.  Add a PACKAGECONFIG for the plugin that is
only enabled for target builds with systemd and explicitly disabled for
native/nativesdk builds, but also keep the explicit disabling of all
plugins as the prioreset plugin also behaves badly inside a build.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>