add --shutdown_after option to recovery

The "--shutdown_after" option causes recovery to power down the device
on completion rather than rebooting.

Removes the last vestiges of the "--previous_runs" argument, which
doesn't seem to be used for anything.

Change-Id: I465eda2ef59d367e2b1c79a8dc69831263c69a4d

Conflicts:
	recovery.cpp