rpl: mute RPL

Removes all but one occurrence of printf and puts within routing (the
remaining puts is called from a debug function.

Possible solution for #641.

I'm not sure if muting all error messages is so smart. Maybe we shoud add a `WARN` and/or `ERR` macro for `DEVELHELP`?
The line break follows at line 162. But thanks for the hint - should have been a `DEBUG` instead of a `DEBUGF` there.