base-files: don't overwrite model name set by target

The condition is always true due to the literal string followed the
-n test parameter. A model name set by target scripts always gets
overwritten this way.

Change the condition to check for an already existing destination file
as it was before 5e85ae9e ("base-files: fix error message during boot").

Signed-off-by: Mathias Kresin <dev@kresin.me>