I see below option from shell to chnage the boot sequence to USB, again I havent tested this , i just found I can configure this.
root@SRX% sysctl -w machdep.nextbootdev=usb
machdep.nextbootdev: nand-flash -> usb
root@SRX%
root@SRX% sysctl -a | grep boot
machdep.nextbootdev: usb ==> next boot dev is USB