travis: make cppcheck check the kernel to see if cppcheck crashes while doing so

adds 2 more jobs (one for clang and one for gcc) which first compile cppcheck (with special CXXFLAGS so it's a bit faster and the travis job does not time out) and then check latest kernel git repo with cppcheck to see if cppcheck crashes while doing so.
some parts of the .travis.yml have been moved as bash scripts into tools/travis/{default,kernel}.sh because travis.yml does not support complex if/else scripting.