Currrently native overrides the object file targets, because it needs
the different include paths to interact with libc and the OS.
This PR simplifies their makefiles to only override the variable
INCLUDES, instead of overriding the targets.
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.
native modules will never need the dynamic INCLUDES, so we define our
own NATIVEINCLUDES. Due to the current make structure, the only way to
not use INCLUDES is to redefine the build rules.
