L4T kernel boots to initrd and then proceeds to filesystem in emmc. You can provide a fake root= in kernel command line while flashing. this will make sure that initrd fails to find filesystem and gives you a prompt on initrd, either on the hdmi console or uart console.
sudo ./flash.sh jetson-tx2 mmcblk1p1
You can pack more tools to the default initrd and erase complete EMMC using the tools or use dd
the img is l4t_initrd.img