am335x_evm: Rework bootcmd to handle two MMC devs

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: Justin Waters <justin.waters@timesys.com>