ARM release build: enable IPv6 SLAAC by default

When building ARM release images, enable IPv6 SLAAC by default in
addition to IPv4 DHCP.

Unlike amd64 (and other desktop/server) releases, ARM releases on SoC
setups are usually deployed by just using the installation image, so
there is no interactive network configuration. Not having IPv6
included by default is kind of an anachronism these days, given that
FreeBSD with the KAME project once pioneered IPv6 technology.

MFC after: 2 weeks