batman-adv: upgrade package to latest release 2020.3

* support latest kernels (4.4 - 5.9)
* coding style cleanups and refactoring
* introduce a configurable per interface hop penalty
* bugs squashed:

  - avoid uninitialized chaddr when handling DHCP
  - fix own OGMv2 check in aggregation receive handling
  - fix "NOHZ: local_softirq_pending 08" warnings caused by BLA

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