[yocto] raspberrypi3-64 target not booting

[Bill Jenkins]

I am trying to just get the simple 64-bit rpi-basic-image to work, but the
Raspberry Pi 3 seems to hang with the rainbow screen at startup.

If I build with MACHINE ?= raspberrypi3, the 32-bit image works just fine.

Is there something special that needs to be set in order for the
"MACHINE ?= raspberrypi3-64" image to boot successfully? 

Thanks,
Bill