The BeagleBone Black can boot from either the MMC card
or eMMC chip on board. We should try both interfaces.

This modification also allows a graceful fallback if
a device exists but boot images are not present on it.

Changes for v2:

* Fix boot partition - it should always show up as mmcblk0p2
* Fix missing FDT load
Signed-off-by: NJustin Waters <justin.waters@timesys.com>