batman-adv: Add orig_interval mesh genl configuration

The OGM packets are transmitted every orig_interval milliseconds. This
value can be changed using the configuration interface.

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the u32 BATADV_ATTR_ORIG_INTERVAL
attribute.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>