Short-circuit looping for audio in Sandbox U-Boot builds

Sandbox U-Boot has no sound, so there is no point in looping while it
doesn't really play.

BRANCH=none
BUG=chromium-os:32603
TEST=Sandbox U-Boot has no sound, no need to loop
Signed-off-by: Taylor Hutt <thutt@chromium.org>

Change-Id: I5ff987d9d80c2d24ae6502c36cf399e582dff524
Reviewed-on: https://gerrit.chromium.org/gerrit/31015
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Commit-Ready: Taylor Hutt <thutt@chromium.org>
Reviewed-by: Taylor Hutt <thutt@chromium.org>
Tested-by: Taylor Hutt <thutt@chromium.org>
1 file changed