Booting CURRENT from PinebookPro SD over 10-RELEASE on eMMC

[Joel <joelp%sdf.org@localhost>]

Hello -- I boot NetBSD 10-RELEASE from eMMC on my PinebookPro. All good 
there -- loving it.
Now I want to try CURRENT. I grabbed a daily HEAD image and used dd to 
write to an SD, followed by the rksd_loader.img for the PinebookPro 
(from pkgsrc) -- the same steps as with 10-RELEASE.
With the imaged SD in the PinebookPro SD slot, it should boot before the 
eMMC. I verified this because that SD card used to have Debian for an 
RPI3 (and of course failed to boot). Disappointingly, that same SD now 
with CURRENT won't boot. I think I'm about to learn some details about 
u-boot etc :)
I tried to catch the messages when u-boot starts but couldn't see 
anything obvious. I couldn't figure out how to break into the u-boot cli 
and diagnose.
I then tried the same CURRENT image with a USB memory stick on the USB2 
port (right side) and still no luck booting. I know booting this way 
works because I can still boot Manjaro Linux (before the eMMC boots) 
that I have on another USB memory stick.
Can someone help me understand why I can't boot another image of NetBSD 
on the PinebookPro SD or USB while I have NetBSD 10-RELEASE on the eMMC?
Specifically, can someone guide me on the right u-boot cli commands to 
diagnose?
Thanks - Joel