Preserve file timestamp via cp --preserve=timestamps.

- Preserve file timestamp to aid debug. (ex crash file creation time)

BUG=None
TEST=Manually executed on samus

Change-Id: I90bef36011cca2601c41402febfd354c88bc2b5d
Reviewed-on: https://chromium-review.googlesource.com/329369
Commit-Ready: danny chan <dchan@chromium.org>
Tested-by: danny chan <dchan@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>