[FS#472] 6to4 support with 1:1 nat

[LEDE Bugs]

The following task has a new comment added:

FS#472 - 6to4 support with 1:1 nat
User who did this - bolvan (bolvan)

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Imagine wan has ipv4 "10.1.2.3". Its mapped 1:1 by isp to 190.1.2.3.
I specify option "ipaddr" as 190.1.2.3.
6to4.sh correctly calculates 2002:BE01:0203::1 but then it sets up link as

ip tunnel add tun6to4 mode sit ttl 80 remote any local 190.1.2.3

instead of correct

ip tunnel add tun6to4 mode sit ttl 80 remote any local 10.1.2.3

And it does not work.
In my patch I force actually present IP to netifd.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=472#comment1637