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Previously, eth0/eth0.1 was used as LAN by default, then any other
interface as WAN. For those devices not matching this a specific hook
was needed.

By using /etc/board.json information, now the LAN/WAN roles are
assigned the same way as OpenWrt does it, so no hooks are needed to
avoid roles swap.

Fixes #214.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>