Revert "update_kernel: Learn arch from board build"

This reverts commit 3476ad8561f5930f84cbcc9e5e49bb421793e893, since it
breaks usage without a full .../build/images/${BOARD}/latest/boot.desc.
We previously were able to build just the relevant package(s) (e.g.,
kernel) and run update_kernel.sh without build_image.

BUG=chromium:997033
TEST=setup_board --board=$BOARD; emerge-${BOARD} chromeos-kernel-X_Y;
     ./update_kernel.sh --remote=${HOST}

Change-Id: I67ece6d3611ca121bec3aae8b6215d9ba280e0f9
Reviewed-on: https://chromium-review.googlesource.com/1767417
Tested-by: Brian Norris <briannorris@chromium.org>
Commit-Ready: Brian Norris <briannorris@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
2 files changed