luci-mod-network: allow disabling interface->device option migration

Introduce a `migrate` properties which selectively allows disabling the
`config interface` to `config device` migration logic for single options.

Use the new flag to disable migration of the "ipv6" option which has
different semantics in interface and device sections.

Ref: https://forum.openwrt.org/t/pppoe-disable-ipv6/92548
Signed-off-by: Jo-Philipp Wich <jo@mein.io>