configs: a64-olinuxino-emmc: add eMMC boot part config commands

mmc bootbus and partconf commands are needed in order to be able to
configure booting from separate boot0/boot1 eMMC partitions.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
[jagan: rebase on master]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>