batman-adv: BATMAN_V: aggregate OGMv2 packets

Instead of transmitting individual OGMv2 packets from the aggregation
queue merge those OGMv2 packets into a single one and transmit this
aggregate instead.

This reduces overhead as it saves an ethernet header and a transmission
per aggregated OGMv2 packet.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>