Maskrom: How to tell what's on the SD card?

[zogkarndon]

I managed to corrupt my EMMC, and am trying to boot from SD.

I've flashed an image onto a 16 GB SD card.

I've set up the debug adapter, and positioned the switch to the left (labeled Maskrom/SD).

However, when I boot, the serial console always shows that it's trying to boot from MMC.

If I interrupt the boot process, I end up at the maskrom console.

How can I check the status of the SD card from the maskrom console?   

Alternatively, how can I force booting from the SD card (from the maskrom console)?