[unit] to overwrite it is enough to copy

Overwrite option means existing data should be overwritten even if
existing data is newer. If there are some other files they should be
untouched.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
Merge pull request #11 from nemomobile/do-not-remove-tree-on-overwrite

[unit] to overwrite it is enough to copy