> you'll need the uboot sd image that Richard put together
> (/n/sources/contrib/miller/uboot.img)

No, that file is just an executable binary of uboot.  An SD image
for pxe-type loading  is /n/sources/extra/pi.uboot.sd.img.gz as
mentioned in booting(8).

I just tried this to check it still works, and found that you
need a vgasize= definition in the /cfg/pxe/NNNNNN file if you're
booting with uboot and want an HDMI screen.  I'm not sure that
was documented anywhere. If you just boot a 9pi kernel from the
SD card without using uboot, vgasize= isn't needed because the
screen size can be detected automagically.