onie-boot-init: (re)-init the ONIE-CONFIG partition

Added a new command that (re)-initializes the ONIE-CONFIG partition.

Using this command wipes any configuration from ONIE-CONFIG and
restores it to a pristine state.

This command is also used by the ONIE self-update installer to
initialize the ONIE-CONFIG partition.

The command does the following:

- destroys the file system and re-formats the partition

- seeds the fresh file system with files from the read-only ONIE-BOOT
  partition.