Init: Add support for mounting emmc by name

Allow mount to recognize partition 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