lint: reduce scope (2)

Follow up of #1565 

I was faster than I thought: for the RIOT "internal" stuff this PR should do it.
I'm not quite sure if it is the right way for the "external" stuff (´dist/tools/linux-border_router´ and ´boards/msba2-common/tools/src´) - Personally I would prefer to leave these files as untouched as possible. The alternative way I can think of is creating a suppression file for cppcheck that includes the "external" tools).

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