faft: Only reimaging the USB and internal disk OS on the first test

Only do it on the first test. Doing it on every test is not necessary and
wastes time.

BUG=chromium-os:34392,chromium-os:34400
TEST=run the following test with reimaging, see only the first test do so:
$ ./run_remote_tests.sh --board daisy --remote dut -a \
    "image=/path/to/chromiumos_test_image.bin" control.faft_lv1

Change-Id: Ia8b2c92fc1f2800c4ceecefe7daf9c89a355f8a9
Reviewed-on: https://gerrit.chromium.org/gerrit/36313
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
1 file changed