Add support for mounting emmc partitions by name

Allow init to mount partitions by their labels on emmc devices. This brings it in line
with mtd devices who already support this.

This initial implementation respects the BOARD_VOLD_MAX_PARTITIONS build flag,
but is limited to partitions on mmc block zero.

Change-Id: I7b30e8beb7d159d920c9eb87d7b9e42094da2649