Split arduino run mode depending on active USB connection

Question

Ok, so i have a unit that i want to add information to the EEPROM, using the USB port, such as SSID for wifi shields.

Then i want to disconnect the USB cable, plugin a battery and then it should run its normal loop.

So something like:

IF usb-cable 
   run_setup_loop();
ELSE
   run_main_loop();