unit-test: Add ability to skip cppcheck

Now you can run openbmc-build-scripts/run-unit-test-docker.sh with
`NO_CPPCHECK=1` to skip cppcheck

We need this mechanizm in case cppcheck hangs. This problem can occur
when compiling with the BOOST_NO_RTTI and BOOST_NO_TYPEID flags.

Tested:
- Run `run-unit-test-docker.sh` as before, verify cppcheck runs
- Run `NO_CPPCHECK=1 run-unit-test-docker.sh`, verify cppcheck is
skipped

Change-Id: Ib1b246dc932b3b36334bfbe51934c206bdeb936f
Signed-off-by: Ewelina Walkusz <ewelinax.walkusz@intel.com>
2 files changed