ag_any: respect target root also when the file does not exist in the original root

[boo#903747](https://bugzilla.opensuse.org/show_bug.cgi?id=903747).

Targeted for SLE12 because it is a trivial bugfix.
The reported bug would only be fixed if the fix is applied to the inst-sys (as a DUD or as a DVD respin).
so at least document it. Ideally with note that it should be fixed and should not go to master :)