Trouble booting RELENG 13.0 on a Zynq XC7Z010 (Zybo-like) board

Hi Everyone,

I am trying to get FreeBSD RELENG 13.0 to boot on a custom Xilinx Zynq
(XC7Z010) board, based loosely on a Zybo.  I am using a custom boot
loader that loads and launches the kernel.

I figured out how to do this back in 2016 with FreeBSD 10 and 11, and
the product that uses this load has been working well ever since.
Back then I used Thomas Skibo's examples as a reference, but those
have not been updated for 13.0.

Just to start with, I have compiled a 13.0 kernel using a slightly
modified ZEDBOARD config, and am attempting to boot that with my
existing 11.0.1-compatible bootloader, and 11.0.1 dtb file.  I
modified the bootloader to jump to 0x02000200 instead of 0x02000100.

Tracing through, I can see that it is panicking in
platform_probe_and_attach(), with the message "No platform module
found!".

Also, incidentally, it doesn't seem to be printing messages to the
serial port like it used to.

Could anyone tell me what a platform module is and where it is set?

Also, where is the ubldr (or equivalent) code that sets up the
environment for the kernel launch now located?

Thanks,

Lee