The "real time" kernel patches are included in Igor's Armbian build tools, but are disabled by default.
Enabling them are simple. (Thanks Igor)
Build "real time" kernel images by doing the following:
apt-get -y install git
git clone https://github.com/igorpecovnik/lib--depth 1
cp lib/compile.sh .
mv ./lib/patch/kernel/sun7i-default/0014-patch-3.4.108-rt136.patch.disabled ./lib/patch/kernel/sun7i-default/0014-patch
You can find a 4.11.9 rt patch here: https://www.kernel.org/pub/linux/kernel/projects/rt/4.11/older/
I have a vanilla mainline kernel with that patch applied and it works for me. I think armbian uses a separate github directory, so you might need to tweak the patch.