It does not make sense to do the static tests in a diferent image than
the compilation tests.

This commit updates the travis config to use the riotbuild image (the same
as murdock). This should fix the issues with old doxygen versions reporting
issues that are not, and also simplify the CI maintainance.

this ensure no python2 flake8 will be installed and prevent confusion with required python3 flake8

Change Travis configuration for cppcheck to fail on new files and keep on
warning about existing ones.

Failures are ignored for now.

This should speed up the installation process a bit.

installing the c++ headers when installing the stock cross-compiler

Some Travis CI machines have 32 CPUs. This sets our concurrency level to 33.
Travis CI kills our buildtest for obvious reasons.

This PR limits the concurrency level to 8 on Travis CI.

It can happen that two PRs don't interfere with each other in the sense
that they hurt automatic rebasing, but still do not work together. The
prime example is PR1 changes the API of some function that PR2 uses. If
PR1 is merged, and PR2 is not rebased before merging, the error might
get unnoticed before the next build. Travis CI would have have told both
PR1 and PR2 are fine, but will complain (rather unnoticedly) that our
master does not compile.

This PR automatically rebases the PR on top of the current master,
before running the tests. If the automatic rebase fails, then this is
fine, because you will need to manually rebase again before merging,
anyway. The manual rebase, i.e. new push, will trigger Travis CI.

So, the main idea of this PR is that to can hit the "Restart Build"
button in Travis CI before hitting the merge button in Github.