disable FsWarn when abroot pkg is used

[taukakao]

If you install gnome-console for example, or remove any package, you will be met with FsWarn.
This is because for one gnome-console changes the default terminal binary, and removing a package means that the executable can't be found.

I think the most reasonable fix would be to just disable FsWarn if packages are added or removed. It should probably just check in the back script if the /etc/abroot/packages.* files exist and are not empty.