Add a UCI setting gluon.mesh_batman_adv.hop_penalty

Example UCI commands:

    uci set gluon.mesh_batman_adv=mesh_batman_adv
    uci set gluon.mesh_batman_adv.hop_penalty=20
    uci commit

`/etc/config/gluon` config section:

    config mesh_batman_adv 'mesh_batman_adv'
    	option hop_penalty '20'

Fixes: #1942

While we're at it, also slightly optimize proto_gluon_bat0_renew.

The new routing_algo site.conf value BATMAN_IV_LEGACY is introduced. With
these changes, the routing_algo setting becomes mandatory.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

[Matthias Schiffer: move to proto_gluon_bat0_setup() and default to BATMAN_IV]

We now create bat0 and primary0 independently of the lower mesh interfaces,
making the whole setup a lot more robust. In particular:

- we can't accidentially destroy primary0 because of concurrent setup and
  teardown runs of different interfaces
- bat0 will always exist, even when no mesh interfaces are up (e.g. no link
  on wired mesh)
- interfaces going down and up again will never tear down the whole of
  batman-adv
- we can enable and disable bat0 independently of the lower interface
  states