batman-adv: add wrapper function to throw uevent in userspace

Using throw_uevent() is now possible to trigger uevent signal that can
be recognised in userspace. Uevents will be triggered through the
/devices/virtual/net/{MESH_IFACE} kobject.

A triggered uevent has three properties:
- type: the event class. Who generates the event (only 'gw' is currently
  defined). Corresponds to the BATTYPE uevent variable.
- action: the associated action with the event ('add'/'change'/'del' are
  currently defined). Corresponds to the BATACTION uevent variable.
- data: any useful data for the userspace. Corresponds to the BATDATA
  uevent variable.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>