[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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6in4 works from the box because 6in4.sh does not have

test_6to4_rfc1918 "$ipaddr" && {
 proto_notify_error "$cfg" "INVALID_LOCAL_ADDRESS"
 return
}

6in4 does not need to find out actual external ip address.
6to4 need it because its used to construct 2002:
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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#comment1639