[FS#360] Regression: no ipv6 with multiple interface

[LEDE Bugs]

The following task has a new comment added:

FS#360 - Regression: no ipv6 with multiple interface
User who did this - Hans Dedecker (dedeckeh)

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The used network config to create multiple IP(v6) addresses is not correct as aliased interfaces needs to be used.
Typically the following config will be used to configure multiple IP addresses assuming BRIGDE_V20 is defined as a logical interface

config interface 'A_MAIN'
        option ifname '@BRIDGE_V20' 
        option proto 'none'

config interface 'LAN_V20'
        option proto 'static'
        option ifname '@BRIDGE_V20'
        option netmask '255.255.255.0'
        option ipaddr '192.168.20.1'

config interface 'LAN_V20_6'
        option proto 'static'
        option ifname '@BRIDGE_V20'
        option ip6addr '2a01:e35:2f01:a61::1/64'

config interface 'LAN_V20_6L'
        option proto 'static'
        option ifname '@BRIDGE_V20'
        option ip6addr 'fd02:15b3:9237:20::1/64'

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=360#comment1119