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>

Documentation/ABI/testing/sysfs-class-net-batman-adv

@@ -1,4 +1,11 @@
 
+What:           /sys/class/net/<iface>/batman-adv/elp_interval
+Date:           Feb 2014
+Contact:        Linus Lüssing <linus.luessing@web.de>
+Description:
+                Defines the interval in milliseconds in which batman
+                sends its probing packets for link quality measurements.
+
 What:           /sys/class/net/<iface>/batman-adv/iface_status
 Date:           May 2010
 Contact:        Marek Lindner <mareklindner@neomailbox.ch>
@@ -12,4 +19,3 @@ Description:
                 The /sys/class/net/<iface>/batman-adv/mesh_iface file
                 displays the batman mesh interface this <iface>
                 currently is associated with.
-