build: allow mounting by partition name through edify

With more devices migrating over to emulated storage there is a need for
increased flexibility when mounting partitions. Edify scripts currently lack
the ability to mount emmc partitions by their labels, so busybox is used
as a reliable alternative.

This allows partitions to be mounted by name while still maintaining
compatibility with older recoveries.

To use define TARGET_SETS_FSTAB in the device's make files

Change-Id: I905740dd566f451b7ac48579a85e1b39a8cfd31a