Endodontists do root canals and a few related procedures. General dentists do exams, fillings, crowns, bridges, maybe root canals, maybe implants, implant restorations, etc, etc.
The ADA considers it unethical to limit your practice or to advertise as a specialist unless you have completed an accredited specialty program. Most states consider it illegal.
Finally, you can't limit your practice to root canals unless you are working off referrals. How many GP's are going to refer root canals to another GP? Probably not enough to keep you busy full time.