target: ensure target/ dir is writable

If the source target skeleton is read-only (eg. because Buildroot's
source dir is), modifications to the output target (such as creating
/etc/hostname and /etc/issue) fail.

(This can happen if the Buildroot source dir is NFS-mounted read-only
to be shared between different machines, for example).

Signed-off-by: Nathan Lynch <ntl@pobox.com>
[yann.morin.1998@free.fr: we use rsync now, not cp; --chmod=Du+w
 suggested by Arnout; clarify commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>