mount_gpt_image.sh: create stateful dirs on demand

If the stateful dir gets wiped (for whatever reason), the mount
script fails:
mount: /tmp/m/usr/local: special device /tmp/s/dev_image does not exist.

BUG=chromium:955147
TEST=mount_gpt_image.sh no longer fails when the stateful dir is wiped

Change-Id: I9b418f45ee43687b05e627ed6e709530db631c51
Reviewed-on: https://chromium-review.googlesource.com/1700158
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
1 file changed