futility: updater: add new quirk 'external_flashrom'

The new quirk will allow the updater to easily switch the underlying
flash driver between the libflashrom and the external flashrom.

BUG=None
TEST=build and run test
BRANCH=None

Change-Id: I5b9da373b1e22a948e04d844973a5683adffe63c
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3534489
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
3 files changed