am335x_evm: Add support for eMMC environment

Some boards, such as the BeagleBone Black, have an eMMC chip intstead
of NAND. We can use the eMMC boot partition to store the environment,
since it isn't used for anything else. This allows us to have a
configurable environment on those boards.

Signed-off-by: Justin Waters <justin.waters@timesys.com>