Hi Larry,
typically your manufacturer already has such devices as part of its production line, and all what you need to supply is the S19, Bin or Intel-Hex file. When we give boards to manufacture from our projects at the university, it is that case. I know that many are using either a Cyclone from P&E or SEGGER Flasher (see Flashing many ARM Boards without a Host PC | MCU on Eclipse): they do not need a host PC and have the binary/image on the flasher. They have as well a 'secure' flashing mode (you can provide the needed keys/certificates) in case there is a concern that too many devices might be programmed (this is a concern in certain areas where you want to avoid that there are 'side' products produced).
Bottom line: check with your manufacturere what he is using and what he needs from you. But in any case, a s19, bin or hex should do it.
I hope this helps,
Erich