make: [dist?]clean

Add [dist?]clean target for the RIOT root directory that cleans out all compilation products.

Also: make clean target from Makefile.include silent and failsafe
Depends on what you want to do. The pattern
```
-@for …
more_command
```
would stop the loop as soon as the first error was encountered, but proceed to `more_command`.
I think in this case we could simply omit the `|| exit 1`, i.e. ignore silently errors.