How to change rootfs from official RPI3 image

[Ricky Zhang]

Hi,

I dd a FreeBSD 12.0 RPI3 image to a SD card. Everything works like a charm
on my RPI3.

But I want to switch rootfs to a larger USB hard drive. The boot partition
contains one configurable text file config.txt. I can't google how the
uboot load kernel from the rootfs partition. To my surprise, the FreeBSD
kernel is in the rootfs partition.

Is this u-boot pieces open source? Where are the user guide documents? The
only document I could find provides no technical details in boot sequence.

thanks,
Ricky