ext-toolchain: fix ct-ng as external rsync issues

When using a CT-NG toolchain as external toolchain (not with the
integration) and said toolchain has u-w directories rsync fails.
So instruct rsync to chmod u+w directories in the target copy to avoid
the issue.

[Peter: notice that this is a workaround for a rsync 3.0.8 bug]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>