init: create symlinks to mtd block device nodes

Given a device in the form of "/devices/virtual/mtd/mtd0/mtdblock0":
* Creates symlinks for mtd block devices in
  /dev/block/mtd/mtd<partition number>
* Creates symlinks based on partition name in
  /dev/block/mtd/by-name/<partition name>

Change-Id: Id8d61be88935a0ef83297d1d5e453d8ba0d586de