Adding and correcting description/rational on why certain cppcheck
warnings or errors are intentionally suppressed.

Testrunner is now impported as a package found in PYTHONPATH, so
import can be placed at the top of the script as usual.

os package is imported by every test script but only used by
a few, thus flake8 check reported errors.

Remove now obsolete sys.append from all tests, as testrunner was moved
to dist/pythonlibs as proper package.

Follow up to #9067 and part of #8821

E402 module level import not at top of file

expected 2 blank lines after class or function definition, found 1

 - Remove BOARD_WHITELIST
 - Clean up CXXFLAGS
 - Clean up CFLAGS
 - Blacklist boards where the C++ library is too large on Travis

These headers do not provide full stl functionality,
but a small subset:
* thread and this_thread
* condition_variable (some timed functions are missing)
* mutex, lock_guard and unique_lock