init/clobber-state: improve factory wipe speed
init/clobber-state: improve factory wipe speed

mkfs is a disk i/o bound program while firmware wiping is SPI/EC I/O bound.
So we can run the two wiping in parallel.

Before this CL:
 wiping on 16G ssd takes 80 seconds to complete
 (30s for boot+mkfs, 60 for firmware wiping)

With this CL:
 wiping totally takes 54 seconds

Apply this CL with http://gerrit.chromium.org/gerrit/#change,333:
 wiping totally takes 30 seconds

BUG=chromium-os:14942
TEST=echo "factory fast" > /mnt/stateful_partition/factory_install_reset
     chromeos-firmware --force --mode=todev
     # measure wiping time

Change-Id: Ifb7c976cbed39ba3c82e145050f45c7a3c813ba5
Reviewed-on: http://gerrit.chromium.org/gerrit/336
Reviewed-by: Stefan Reinauer <0ab4fef0c228799b0c44e01c732054569b690b58@chromium.org>
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

joshfire/chromiumos-platform-init