Update to out of source makefile build and add run-cbmc-proofs.py
CBMC proofs can now be run with Litani with the command
"./run-cbmc-proofs.py"
Based on commits:
* 1646301 - Ignore CBMC proof failures, fail the build later (4 months ago) <Kareem Khazem>
* 7e8c91a - Fix Makefile prerequisite symbol for CBMC proofs (4 months ago) <Kareem Khazem>
* bee04be - Enable CBMC proofs to run in CI (4 months ago) <Kareem Khazem>
Found in https://github.com/FreeRTOS/FreeRTOS-Plus-TCP
