6lowpan: fix/move/cleanup debug functions

There are several issues on current debug behaviour.
This patch fix the following issues:

- Fix debug printout only if DEBUG is defined.
- Move debug functions of 6LoWPAN code into 6lowpan header.
- Cleanup codestyle of debug functions.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>