Make: exterminate 'clean' buildtarget

Closes #993.

We do not need to descend into the modules to know what to do on
`make clean BOARD=blub`. We can just invoke `rm -rf bin/blub`.

This PR only keeps the descending into the USEPKGs, since they might
want to delete cached/downloaded/extracted data.
Add missing doxygen for pthread funs and structs
pthread: fix spinlock
Merge pull request #1017 from Kijewski/issue-980

documentation: Add missing documentation for pthread functions and structures
Merge pull request #995 from Kijewski/issue-993

Make: exterminate 'clean' buildtarget clutter
pthread: implement reader/writer lock
Add pthread_rwlock test
Merge pull request #1018 from Kijewski/pthread-rwlock

pthread: implement reader/writer lock