devtool: deploy-target: preserve existing files

devtool: deploy-target: preserve existing files

If files would be overwritten by the deployment, preserve them in a
separate location on the target so that they can be restored if you
later run devtool undeploy-target.

At the same time, also check for sufficient space before starting the
operation so that we avoid potentially failing part way through.

Fixes [YOCTO #8978].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>