We have a method where we can boot into fog from a usb stick. Its mainly used for debugging purposes (and MAC computers) but it should work for your request.
Here are the instructions for setting this up: https://forums.fogproject.org/topic/7727/building-usb-booting-fos-image
Pay attention to the requirements and the caveats of booting this way. You will loose some functionality of FOG that is delivered through the iPXE boot menu, but imaging is surely possible booting from a usb stick.
Look at the forum chat bubble for a bit more info.