Allow mapping of raw block devices to partition names

Allow mapping of raw block devices (those without a partition table)
to partition names for first stage mount and ueventd.  For example, a
block device `vdb` that contains only an ext4 metadata partition could
be mapped to `metadata` such that /dev/block/by-name/metadata is
created.

The mapping is provided by the `androidboot.partition_map` bootconfig
or kernel command line parameter.

Bug: 159943320
Test: boot with raw vdb,metadata;vdc,userdata partitions
Change-Id: Ib6a939d6fb88b85c46c81d613425a127627a734b