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

BUG=None
TEST=Time update_kernel and see savings; verify that kernel still gets
updated.

Change-Id: I5afead3e70720ee6dc9e6a822ef1a98032d93b79
Reviewed-on: https://gerrit.chromium.org/gerrit/39783
Commit-Ready: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>
Reviewed-by: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>
Tested-by: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>

sigma/coreos-scripts,nicescale/scripts,fivethreeo/scripts,nicescale/scripts,endocode/scripts,vmware/coreos-scripts,trnubo/scripts,marineam/coreos-scripts,sigma/coreos-scripts,mischief/scripts,smilart/scripts,BugRoger/scripts,glevand/coreos--scripts,sigma/coreos-scripts,gtank/scripts,fivethreeo/scripts,glevand/coreos--scripts,mjg59/scripts,zhang0137/scripts,andrejro/coreos--scripts,BugRoger/scripts,zhang0137/scripts,endocode/scripts,endocode/scripts,gtank/scripts,trnubo/scripts,trnubo/scripts,crawford/scripts,nicescale/scripts,crawford/scripts,coreos/scripts,andrejro/coreos--scripts,bcwaldon/coreos-scripts,crawford/scripts,mjg59/scripts,marineam/coreos-scripts,vmware/coreos-scripts,mischief/scripts,BugRoger/scripts,marineam/coreos-scripts,zhang0137/scripts,gtank/scripts,smilart/scripts,bcwaldon/coreos-scripts,mjg59/scripts,coreos/scripts,andrejro/coreos--scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,smilart/scripts,fivethreeo/scripts,mischief/scripts