Expand the existing run mmcboot command into a clear set of parameters which can be modified if the partition layout changes in the future.

http://git.denx.de/?p=u-boot.git;a=blob;f=include/environment/ti/boot.h;h=799d9840aa2ebf0c8d2dc6d6150fae0356cac595;hb=HEAD#l38

Migrated from https://projects.linaro.org/browse/LAVA-1152