common.sh: adapt to get clean shellcheck BUG=chromium:1141907 TEST=unit tests and manual signing run Signed-off-by: George Engelbrecht <engeg@google.com> BRANCH=none Change-Id: I39b133ca69e717576140b418fc59dd167f068d59 Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2514558 Reviewed-by: Sean McAllister <smcallis@google.com> Reviewed-by: LaMont Jones <lamontjones@chromium.org> Commit-Queue: George Engelbrecht <engeg@google.com> Tested-by: George Engelbrecht <engeg@google.com>