MFC r339465: rc.initdiskless: add support for auxiliary NVRAM.

MFC r339465: rc.initdiskless: add support for auxiliary NVRAM.

  Currently, rc.inidiskless assumes that local system configuration
  changes are kept in some mountable file system. For example,
  nanobsd uses dedicated partition mounted as /cfg for this.

  However, small embedded devices like MIPS routers may have no enough flash
  space to keep full-blown file system but have only one or couple
  small flash blocks to keep persistent local configuration overrides.

  This change extends rc.initdiskless and introduces ability to run auxiliary
  command /conf/T/M/extract that is supposed to extract configuration overrides
  from such local storage.

  For example, the command /conf/default/etc/extract may contain something like:

  cd "$1" && bsdcpio --quiet -idu < /dev/map/cfg

  bsdcpio command extracts compressed archive from the storage to /etc
  assuming the storage is exposed by the kernel as /dev/map/cfg to userland.

PR:		204215