Fix bug in gooftool.

1.Fix FindScript() In gooftool. When running gooftool,
sys.path[0] is py directory, but scripts are in sh directory.
Fix the path in Findscript to search files under sh directory instead of
py directory.
2.Fix GetPrimaryDevicePath(). The function returns mmcblk05
instead of mmcblk0p5. Fix the logic so it will not return mmcblk05

BUG=chrome-os-partner:8812
TEST=run /usr/local/factory/gooftool prepare_wipe on DUT, will not fail at finding scripts or
finding rootfs partition.

Change-Id: Iaa974cb199a541d261f6320b27bde259e1ccbc95
Reviewed-on: https://gerrit.chromium.org/gerrit/23462
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>