[MDC-524] simetbox-openwrt-core: lac-bcop-1 FR-15: enable bcp38 filte…

[MDC-524] simetbox-openwrt-core: lac-bcop-1 FR-15: enable bcp38 filtering by default

Enable bcp38 at initial configuration.  This avoids the trap of
force-enabling it on sysupgrade, which we'd have to work around if using
uci-defaults.