fat16copy: Allow to copy files to an existing directory.

This patch re-uses existing directories when copying directories,
allowing to copy files to a directory previously created.

Bug: 34811400
Test: fat16copy.py fat.bin a/b/foo; fat16copy.py fat.bin c/d/foo

(cherry-picked from 0af24a27a8d0dc7e1782f36a5a9963021ff8b3f6)

Change-Id: I283184da18f3c4951d6c84aafa4185074791acd2