Add legacy updater to updater{3,4,5}.sh.

This allows chromebooks which shipped with non functional
legacy boot systems to be fixed remotely, without requiring
reflashing or custom payloads to be compiled.

BUG=none
TEST=Tested on zako chromebook manually, with autoupdate mode

Change-Id: I43d86a5410b6a6a987f394fd978631b5ca655a51
Reviewed-on: https://chromium-review.googlesource.com/259860
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Colin Rice <clr@chromium.org>
Commit-Queue: Colin Rice <clr@chromium.org>
4 files changed