This commit simplifies the regression test for the doxygen build to
allow only 0 warnings.
Clean doxygen.runlog and doxygen.runerr for clean target
and ignore them in .gitignore.
Parts of the stm32w108 doxygen comments have explicit links to symbols that do not exist anywhere in our source base, let alone be documented. This is likely to be caused by a partial import of manufacturer libraries in the Contiki source tree.
These links were previously not generating warnings in the doxygen log because we were not defining `DOXYGEN_SHOULD_SKIP_THIS` and they were thus being skipped altogether by the doxygen pre-processor. Defining `DOXYGEN_SHOULD_SKIP_THIS` causes those doxygen comments to get processed and to thus generate warnings.
This commit removes explicit links to non-existent symbols and updates `doxyerrors.cnt` accordingly.
