image_to_live: clean up subprocesses; run tail -f in pty
image_to_live: clean up subprocesses; run tail -f in pty

This kills all subprocesses when the script dies. Also, tail -f is
forced to run in a pty so that it uses line buffering.

BUG=chromium-os:20785
TEST=manual test

Change-Id: If02074f37043e612789515cdf5d26ac9dd45f02a
Reviewed-on: http://gerrit.chromium.org/gerrit/8239
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: Andrew de los Reyes <7bd5f9be8af0e6bb22e5665635eff3a09edb828c@chromium.org>

mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util