classes/package_rpm: use weak user/group dependencies

classes/package_rpm: use weak user/group dependencies

rpm 4.19 automatically generates provides and depends for user and
groups:
https://github.com/rpm-software-management/rpm/blob/rpm-4.19.x/docs/manual/users_and_groups.md#dependencies

This mechanism relies on sysusers.d for the 'provides'
part, and thus is systemd-only at best. So we need to disable it for now,
otherwise image generation fails with unresolved dependencies.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>