update_kernel: Save .5 seconds by avoiding tmp copy for kernel image

There's no need to store to a temporary file when updating the kernel
partition.  Just stream straight to the right partition.  This appears
to save about .5 seconds.

TEST=Time update_kernel and see savings; verify that kernel still gets

Change-Id: I5afead3e70720ee6dc9e6a822ef1a98032d93b79
Reviewed-on: https://gerrit.chromium.org/gerrit/39783
Commit-Ready: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
1 file changed