The boot device is chosen by cboot for NX. I am not very sure if there is an option for you to choose the boot device.
Could you check your serial console log?
I am more familiar with uboot case so take uboot for example, uboot would check mmc0 (emmc) and mmc1 (sdcard) when it tries to load kernel.
If it finds kernel on mmc0, it will load it. Please note that kernel file could be separate from rootfs.
One case is the “rootfs on usb” on Nano and TX2. The usb controller in uboot does not work on these platforms. But it can still mount the rootfs on usb. The reason behind this is because the uboot loads the kernel from emmc and uses kernel to load the rootfs on usb drive.
Thus, back to your question on NX.
First, whether cboot gives you an option to choose mmcblk0p2?
Second, if kernel is able to load rootfs from mmcblk0p2?