Fix #361, add option for trailer bytes in CFDP PDUs

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
Adds an option to insert platform-specific padding at the end of CFDP PDU encapsulation. The padding area may be utilized by the deployment to add arbitrary verification information to the PDU.

Fixes #361

Testing performed
Build and run all tests
Run a CFDP file transfer

Expected behavior changes
None with default config. New config option allows user to add padding to the end of CFDP PDUs for trailer bytes.

System(s) tested on
Debian

Additional context
The extra space is added only to the SB encapsulation of CFDP PDUs, this is not within the CFDP PDU itself (that is, the size inside the CFDP PDU does not include these extra bytes, but the size in the CFE SB message that holds the PDU does include it).

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[chillfig]

Looks great!