cros_bundle_firmware: Move uboot handling code closer together BUG=chromium:595715 BRANCH=none TEST=none Change-Id: I0c4321473bba06319a4553f8fb168ef04e7902a0 Signed-off-by: Patrick Georgi <pgeorgi@google.com> Reviewed-on: https://chromium-review.googlesource.com/397619 Commit-Ready: Patrick Georgi <pgeorgi@chromium.org> Tested-by: Patrick Georgi <pgeorgi@chromium.org> Reviewed-by: Stefan Reinauer <reinauer@chromium.org>