This implementation is optimized for a little code and data size, not
for speed. IMO the code is more readable than in the reference
implementation.
The biggest advantage of ChaCha over other stream ciphers is the very
little data usage with only 64 bytes of context, and its good encryption
speed.
Also part of this PR is pseudo-random number generator, that just
returns the keystream of a randomly initialized ChaCha context.
For many modules the `Makefile` contains a line like
```
MODULE:=$(shell basename $(CURDIR))
```
This conclusively shows that we do not have to set the module name
manually.
This PR removes the need to set the module name manually, if it is the
same as the basename. E.g. for `…/sys/vtimer/Makefile` the variable
make `MODULE` will still be `vtimer`, because it is the basename of the
Makefile.
