update_kernel: Speed copy for kernel, modules, and firmware copy
update_kernel: Speed copy for kernel, modules, and firmware copy

If possible, we now uses rsync to copy elements from the host to the
device.  Using rsync to copy sped my "no-op" update kernel from 30
seconds down to ~5 seconds.

If rsync isn't available (a non-test build), speed up the fallback
case to:
- Never write tar to disk (on either host or target)--just stream
  straight to/from the network.
- Don't use bzip2 compression.  It's assumed that we've got a fast
  connection to the device so this is overkill and slower.
In this case we're at ~15 seconds for a "no-op" update kernel.

BUG=None
TEST=With old code, run this:
  time ./update_kernel.sh --remote=${REMOTE} --novboot --noreboot
...and see 30 seconds with old code and 4 seconds with new code.
TEST=Use update_kernel for updating kernel.  See that kernel / modules
get updated.

Change-Id: Iafad68221bd459fce5a4c661e07cd649b058d7b9
Reviewed-on: https://gerrit.chromium.org/gerrit/39393
Reviewed-by: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>
Tested-by: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>
Reviewed-by: Mandeep Singh Baines <19acf4ef1f391f4aab404dc5703cac3632e06f3a@chromium.org>
Commit-Ready: Doug Anderson <49983c4a76b998672954ed5edf4a62db4a336502@chromium.org>

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