/boot.config

[Fbsd1]

During the boot process I want to change the device used to boot from.
 From the default 0:ad(0,a)/boot/loader
to 0:da(0,a)/boot/loader forcing the boot to continue from usb stick.

Here is the problem, the bios have no option to boot from USB device.
So thinking let the bios point to first drive to start the boot process 
and have a /boot.config file to redirect to booting from the USB stick.
I am assuming the '0' zero will mean the first USB device.

Is there any command i can use to verify the single USB stick is the 0 
device?

Is this concept valid?