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 <[email protected]> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3534489 Reviewed-by: Yu-Ping Wu <[email protected]> Commit-Queue: Yu-Ping Wu <[email protected]>