batman-adv: Consolidate logging related functions

There are several places in batman-adv which provide logging related
functions. These should be grouped together in the log.* files to make them
easier to find.

Reported-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>