cross-compile:

tar xvf openwrt-sdk*
cd openwrt*
./scripts/feeds update -a
./scripts/feeds install -a
mkdir package/p4xdp
ln -s ../../package/p4xdp feeds/base/p4xdp
cp -r ../misc/native package/p4xdp/
cp ../misc/openwrt-p4xdp/* package/p4xdp/
cd package/p4xdp/native
cd ../../..
make package/p4xdp/compile
ARCH=`ls bin/packages/`
mv bin/packages/$ARCH/base/* ../$ARCH-


on the box:

apk add  gre
apk add --allow-untrusted  p4xdp*


/etc/config/network
config interface 'cpuport'
        option proto 'gretap'
        option peeraddr '10.1.11.12'
        option ipv6 '0'
        option promisc '1'
        option mtu '1450'