vboot/Kconfig: Drop BOOT_EXTERNAL_ON_DEV

With CL:3461454, the Kconfig option BOOT_EXTERNAL_ON_DEV is actually
never used. All the jetstream boards with the config enabled were also
removed in CL:4891362. Therefore, drop the unused config.

BUG=b:151200757
TEST=cq
BRANCH=none

Change-Id: I39f72b78fe67bbe197effb3e8230f60641dd4641
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/depthcharge/+/5485921
Reviewed-by: Yidi Lin <yidilin@google.com>
Commit-Queue: Yidi Lin <yidilin@google.com>
1 file changed