Fully autonomous cars are only “two to three years away,” according Tesla chief Elon Musk. Although the cars will then need to leap the hurdle of regulatory approvals, which the exec and perennial inventor said would tack on another “one to five years.” Tesla has quietly been releasing self-driving features into its vehicles. Its latest software upgrade equips Tesla vehicles with autonomous highway driving. Between 600 and 700 drivers have been testing the feature on…