Phanbell: Always use MMC0 with Fastboot

* Otherwise, during an SD card boot fastboot will attempt to flash the
SD card. This doesn't work as the partition table is different.
* Also enables a better story for recovering the eMMC - any SD card with
u-boot can use fastboot to reflash.

Change-Id: I0ee135696b55a3e168b0a3aece1d49b736ea81ff
1 file changed