realtek: Use firewall4

realtek: Use firewall4

The realtek target is not a router, but basic device, see DEVICE_TYPE.
The basic device type does not come with firewall by default, see
include/target.mk for details. The realtek target extended
DEFAULT_PACKAGES manually with firewall.

This changes the defaults to take firewall4 and nftables instead of
firewall and iptables. This also adds the additional package
kmod-nft-offload.

The only difference to the router type is the missing ppp,
ppp-mod-pppoe, dnsmasq and odhcpd-ipv6only package.

This increases the compressed image size by about 422KBytes.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>