Re: Using usb key
hm I just used auto-install mode for that.
you find that under system->advanced->Usb install in gui
or in config system auto-install in cli.
It is enabled by factory default and on gui it shows you the default filenames (on cli it does not show these unless you explicitely set them).
image.out for firmware image
fgt_system.conf for configuration
if this is on the usb key which is plugged into the fgt and you reboot or power cycle the FGT it will detect that and automagically install firmware and config. If there is only one of them it will install only that one.