building v21.02.0-rc1 for C7 - unfortunately I am getting errors....
/build_dir/target-mips_24kc_musl/linux-ath79_generic/xtables-addons-2.14/extensions/LUA/xt_LUA_target.c:82:7: error: implicit declaration of function 'skb_make_writable'; did you mean 'skb_try_make_writable'? [-Werror=implicit-function-declaration]
if (!skb_make_writable(pskb, pskb->len))
Well I am at a loss how you are building from a 21.02 tree yet manage to use xtables-addons 2.x if that old PR you linked to already talks about 3.9. Sounds like something is messed up on your end. Are you using the right package feeds? Those for 21.02?