realtek: don't implicitly configure port MAC addresses

realtek: don't implicitly configure port MAC addresses

Currently, the 02_network script always configures MAC addresses for
each individual LAN port unless "lan_mac_start" is set to "skip". This
behaviour can be unexpected, and is also somewhat broken, as it even
continues to do so when "lan_mac_start" is empty.

Change it to only do the configuration if "lan_mac_start" is non-empty,
and also remove the fallback to "lan_mac", making this more obvious and
less error-prone.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://github.com/openwrt/openwrt/pull/21644
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>