Many modules have subdirectories. Often these subdirectories should only
be included under certain circumstances. Modules that use submodules
currently need to use this pattern:
```make
DIRS = …
all: $(BINDIR)$(MODULE).a
@for i in $(DIRS) ; do $(MAKE) -C $$i ; done ;
include $(RIOTBASE)/Makefile.base
clean::
@for i in $(DIRS) ; do $(MAKE) -C $$i clean ; done ;
```
This PR moves the `all:` and `clean::` boilerplate into `Makefile.base`.
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.
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.
