OS Ubuntu 20.04 on WSL2, this setting https://openwrt.org/docs/guide-developer/build-system/wsl, Debian/Ubuntu dependencies listed here https://openwrt.org/docs/guide-developer/build-system/install-buildsystem.
Clone mainstream openwrt repo
git clone https://github.com/openwrt/openwrt.git
Add @f00b4r0 patches
git remote add foobar https://github.com/f00b4r0/openwrt.git
git fetch foobar
git merge foobar/hAP-ac2-cleanup
Rebase local repo
git rebase
Update packages
./scripts/feeds update -a && ./scripts/feeds install -a
Configure the build (IPQ40XX / Mikrotik ac2)
make menuconfig
Then build
make -j5 (where 5 is CPU core (4) # +1)
Initial compilation takes like an hour but subsequent builds are fast, make sure to run each time you rebase (aka update local OpenWRT repo)
./scripts/feeds update -a && ./scripts/feeds install -a
make clean
PD: I use https://www.sordum.org/9480/defender-control-v1-6/ to disable WD and avoid slowdowns caused by the AV scanning