This is purely to finally unblock RIOT-OS/riotdocker#104, but may also
be useful to add future exclude patterns.
Exclude file was generated using
```sh
make doc 2> dist/tools/doccheck/exclude_patterns_base
grep "^${PWD}" dist/tools/doccheck/exclude_patterns_base | \
sed -E -e "s#^${PWD}/(.*:)([0-9]+): #\1DIGITS_MAGIC: #" \
-e 's/[]\\.$*{}|+?()[^-]/\\&/g' \
-e 's/:DIGITS_MAGIC:/:[0-9]+:/' \
> dist/tools/doccheck/exclude_patterns
rm dist/tools/doccheck/exclude_patterns_base
```
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.
