Info:
https://github.com/el1n/OpenWRT-package-softether
https://github.com/Alberts00/OpenWRT-Softether-builder/
https://wordpress.tirlins.com/2015/03/setting-up-softether-vpn-on-openwrt/
https://gist.github.com/kacchan822/0f3ad6590a4329134a5b

Steps:

sudo apt-get install -y subversion make gcc g++ libncurses5-dev libghc-zlib-dev libreadline-dev libssl-dev gawk bzip2 patch xz-utils git unzip

git clone https://github.com/openwrt/openwrt
cd openwrt
echo "src-git softethervpn https://github.com/el1n/OpenWRT-package-softether.git" >> feeds.conf
./scripts/feeds update
./scripts/feeds install -a
./scripts/feeds install softethervpn

make defconfig
make menuconfig