image_to_vm.sh: Infer the board name from lsb-release.

If --board is not set in image_to_vm.sh, fetch the board name from
/etc/lsb-release in the image.

BUG=chromium:495382
TEST=`./image_to_vm.sh --from=../build/images/gizmo/latest --test_image`
  works.

Change-Id: Ia75e76543a21b87e140f31ccc9b72e142837e1eb
Reviewed-on: https://chromium-review.googlesource.com/275139
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Steve Fung <stevefung@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
3 files changed