The previous doccheck would give a false negative when doxygen does
not even run (for example, because of misconfiguration).
Also, when doxygen fails to run, print the full output.
The doccheck script reports reports a false positive when executed from
any directory but `RIOTBASE`. With this fix, `make doc` changes into
the currently unused `RIOTBASE` variable.
This is an alternative approach to #7217, which removes this variable,
but keeps the false positive aspect of the script untouched.
