update_kernel: Learn arch from board build

The boot.desc file in a build contains the information about what arch
the build is for. Parse this file for the architecture and pass it along
to vbutil. This avoids SSH to the DUT just to run uname.

TEST=update kernel on Cheza works

Change-Id: Ia9a43dff54943ee18360631b25a35c832743eea5
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1720708
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed