I was able to fix this by mounting the boot partition from the provided xenial sd-card image and examining their boot setup, which includes these two magic lines: fdt addr ${dtb_addr} fdt set mmc${boot_mmc} boot_device <1> "boot_device" is an undocumented (and poorly named?) DTB property. ...

Hi, I've built my own kernel and u-boot as described here: http://wiki.friendlyarm.com/wiki/index.php/Mainline_U-boot_and_Linux When I come to boot the SD card, u-boot passes a "root=mmcblk0" command line parameter to the kernel, but when the kernel boots it enumerates the first mmc device...