Only update walls when exploring

We don't need to update the walls all the time only when exploring
the cell for the first time.
Added debug code and fixed bug in drive_to_target

We still have to check for walls when driving to a target. The
flood number alone will not tell us if we can move to the location.
We still have to check for wall and ensure that it is a valid move.
Removed unused params from motor

We no longer use the nav array and the position so don't use
them.