batman-adv: ELP - adding sysfs parameter for elp interval

This parameter can be set individually on each interface and
allows the configuration of the elp interval for the link
quality measurements during runtime. Usually it is desirable
to set it to a higher (= slower) value on interfaces which
have a more static characteristic (e.g. wired interfaces)
or very dense neighbourhoods to reduce overhead.

Developed by Linus during a 6 months trainee study period in
Ascom (Switzerland) AG.

Signed-off-by: Linus Luessing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
[antonio@open-mesh.com: respin on top of the latest master]
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>