ringbuffer: remove example
ringbuffer: add convenience functions

This patch add `peek` functionality, and empty and full helpers.
ringbuffer: fix parameter types

It is a bad idea to use signed types for lengths.
Mark pointers a `restrict`, since the ringbuffer is not thread safe
anyway.
ringbuffer: remove pointer to the end

There is no need for an explicit pointer to the end of the buffer.
ringbuffer: don't overwrite in `ringbuffer_add`
ringbuffer: add documentation
ringbuffer: add static initializer
tests: add ringbuffer unittest
Merge pull request #1101 from Kijewski/issue-979-2

ringbuffer: versatility and optimizations
Add generic `char` pipe implementation
Merge pull request #935 from Kijewski/add-generic-pipe

sys: Add generic pipe
 core: simplify mutex signatures
core: simplify mutex initializer
core: remove unneeded calls to mutex_init()
Merge pull request #1440 from Kijewski/issue-1436

core: simplify mutex signatures
doc: doxygen: EXTRACT_STATIC = YES
doc: little mutex.h fixes

- a university is not an author
- move internal tags to the right place
Merge pull request #1489 from LudwigOrtmann/minidoc

doc:   #1440 follow up