tools: ynl-gen: use temporary file for rendering

Currently any error during render leads to output an empty file.
That is quite annoying when using tools/net/ynl/ynl-regen.sh
which git greps files with content of "YNL-GEN.." and therefore ignores
empty files. So once you fail to regen, you have to checkout the file.

Avoid that by rendering to a temporary file first, only at the end
copy the content to the actual destination.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>