Add certificate chain file import to TLV 81 and 82 in bootfile encoding

[sergessn]

Hello,

We need to have the ability to set certificate file path/name in "GenericTLV TlvCode 81" and "GenericTLV TlvCode 82" for file import during encoding DOCSIS 3.1 bootfiles. This will help to remove the manual inserting certificates as hex strings to bootfiles as we have now GenericTLV TlvCode 81 TlvLength 254 TlvValue 0x00 ... 00 ;.
It will be good if this will work like certificate import in options ManufacturerCVC and CoSignerCVC.

Please add this feature to "docsis" program.