oe.path: fix copyhardlinktree()

oe.path: fix copyhardlinktree()

The change to preserve extended attributes in copytree() and
copyhardlinktree() (e591d69103a40ec4f76d1132a6039d9cb1555103)
resulted in an incorrect cp invocation in copyhardlinktree() when
the source directory contained hidden files.

This was because the passed src was modified in place but some code
paths expected it to remain unmodified from the passed value.
Resolve the issue by constructing a new source string, rather than
modifying the passed in string.

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>