init: create symlinks to block device nodes

eMMC block device names may change based on the detection order of
the eMMC device and any other SD bus devices, such as a removable SD
card.

This patch adds support to init for:
  * Symlinks to block devices.  When a block device uevent is
    processed, if it starts with "/devices/platform", the platform
    driver name is parsed out, and symlinks to the block device are
    created in /dev/block/platform/<platform driver>/
  * Symlinks based on partition name and number.  If the uevent for
    a block device contains information on the partition name or
    number, symlinks are created under
    /dev/block/platform/<platform driver>/by-num/p<partition>
    and
    /dev/block/platform/<platform driver>/by-name/<partition name>

init.rc can then use a device path like the following to mount an
eMMC device:
/dev/block/platform/<platform>/by-name/system /system ro

Change-Id: Id11bb7cdf1e2ada7752a5bd671cbf87237b34ae2