setup_symlinks_on_root: handle missing dev_image dir

The dev-install integration test will delete this dir and leave it
missing before running `./mount_gpt_image.sh --unmount`.  When that
script tries to clean up the symlinks, it fails because it assumes
the dev_image dir exists.  Add an explicit mkdir just in case.

BUG=chromium:955147
TEST=running devinstall_test.py doesn't die while unmounting

Change-Id: I3fb055ab26227a5d5b96f0717482a58d543cef55
Reviewed-on: https://chromium-review.googlesource.com/1700157
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