batman-adv: Add support for hardif settings

batman-adv: Add support for hardif settings

B.A.T.M.A.N. V support additional settings which are hardif specific. The
batadv_hardif proto has to expose them to allow automatic configuration.
The default configuration would be:

  config interface 'bat0_hardif_eth0'
  	option proto 'batadv_hardif'
  	option master 'bat0'
  	option ifname 'eth0'
  	option mtu '1536'
  	option 'elp_interval' 500
  	# string like '1mbit' is accepted instead of kbit for override
  	option 'throughput_override' '0'

Signed-off-by: Sven Eckelmann <sven@narfation.org>