CHROMIUM: scripts: Allow prepareconfig to accept a destination

CHROMIUM: scripts: Allow prepareconfig to accept a destination

This is necessary to allow kernel sources to not be copied
and the .config file written directly to the build directory.

BUG=chromium-os:30559
TEST=x86, amd64, arm-tegra2 and daisy paladin trybots
  Ran with no destination parameter and confirmed existing behavior
  Ran with destination parameter and confirmed it was written to
Signed-off-by: Jonathan Kliegman <kliegs@chromium.org>
Change-Id: Idd8842cff8756717fd866dbd7a48da16411479b0
Reviewed-on: https://gerrit.chromium.org/gerrit/22148
Reviewed-by: Olof Johansson <olofj@chromium.org>
Commit-Ready: Jon Kliegman <kliegs@chromium.org>
Tested-by: Jon Kliegman <kliegs@chromium.org>