build: include kmod-ipt-offload in default images
Netfilter flow offload has now started to become useful and suitable for
a wider testing audience. Configuring it via UCI is also integrated in
firewall3 by adding 'option flow_offloading 1' to the 'defaults'
section in /etc/config/firewall
Signed-off-by: Felix Fietkau <[email protected]>