mkbootfs: Add support for a dev node list file

Let mkbootfs create a list of dev nodes and directories, based on a
file:

  $ mkbootfs -n node_file initramfs/ > initramfs.cpio

This file is following the same format as the kernel usr/ cpio list:

  $ cat node_file
  # My device nodes
  dir dev 0755 0 0
  nod dev/null 0600 0 0 c 1 5

Bug: 254835242
Change-Id: Ieb75b1d1026a9426581b6f2b3aa692a05722ba0d