SD card device name changed on wandboard quad.

[peter green]

While upgrading a wandboard quad from 4.4 to 4.6 (debian armmp kernels) 
I got a boot failure. I tracked this down to the device name for the SD 
card changing from mmcblk0 to mmcblk2

Any idea why this has changed? There don't seem to be any other 
lower-numbered mmcblk devices.