file_lock: Add fallback directory
file_lock: Add fallback directory

This adds a fallback directory in case SYSTEM_LOCKFILE_DIR is
unavailable. Since this is a band-aid meant to help older systems
auto-update, the fallback path is hardcoded to "/tmp" as to avoid
polluting the overall lockfile API.

BUG=chromium:616620
BRANCH=none
TEST=Tested on veyron_jaq by removing /run/lock and seeing
mosys, flashrom, and ectool run successfully with
firmware_utility_lock in /tmp.

Change-Id: Idbe63a574474ec35a5c3b6fe2b0fb3b672941492
Signed-off-by: David Hendricks <3b0deacc4b5357f519084f1ea34bd42bf5d34545@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/348850
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec