Does the 2015 Retina MacBook have firmware target disk mode?
Yes - you need an USB C data cable to use this and the power cable Apple ships isn't a data cable for these needs, but the firmware and hardware support target move over USB.

Apple's MacBook ports article now has a section titled "USB Target Disk Mode":


  USB Target Disk Mode
  Your MacBook with USB-C port supports USB Target Disk Mode. Enable Target Disk Mode for use with the following USB-C cables:
  1) USB 3.0 or USB 3.1 USB-A to USB-C Cable 
  Target Disk Mode can be used with this cable to manually transfer data from any Mac notebook with USB-A port(s) to your MacBook with USB-C port. This cable cannot be used to migrate data using the Setup Assistant or Migration Assistant application.
  2) USB 3.0 or USB 3.1 USB-C Cable (USB-C to USB-C)
  Target Disk Mode can be used to migrate data from any MacBook with a USB-C port to another MacBook with USB-C port using the Setup Assistant or Migration Assistant application. 
  Note: These cables are not currently available from Apple.
  To enable Target Disk Mode, hold down the T button on your keyboard while starting your MacBook. Then connect either USB-C cable described above. Note that the USB-C Charge Cable (2m) that comes with MacBook does not support Target Disk Mode.Your MacBook supports USB Target Disk Mode when you use it with a compatible, full-featured (“super speed”) USB-C cable. Note that the USB-C Charge Cable (included with MacBook) does not support USB Target Disk Mode.
  
  To enable Target Disk Mode, hold down the T button on your keyboard while starting your MacBook. Then connect the USB-C cable. Use a full-featured USB-C to USB-C cable to connect to another MacBook, or a full-featured USB-A to USB-C cable to connect to a Mac with standard USB-A ports.


Since this section was still available two days ago, and the MacBook launched on April 10, 2015 — I would say the answer is almost certainly yes, but not with the cable it comes with.