Adding support for signing separate efi.bin & dtb.bin

  o Support for signing efi.bin updated
    - efi.bin can be signed with/without dtb directory
    - Make sure there are not many dtbs files in dtb directory due to
      size problem
  o Support for signing dtb.bin added
  o Updated README.md file
  o Updated typos in config.ini file

Change-Id: I179706a0f44b0a57d09a972f7e5eb019333aa583