OK, I have tried to follow quite a few different "solutions" here, but so far no joy - and not much hair left, so hoping someone has the right "recipe" to make this work. Here is the short version of where I'm at now,
1) Installed rpi-update, rebooted, then installed rpi-source (reboot is important!). Running 4.4.18 kernel, and /lib/modules/4.4.18-v7+/build does point to /home/pi/linux/ ... so this part is working. BTW, I can fully build the kernel, all good.
2) Now, I try to build the EW-7811UTC (rtl8812au) driver, and that's where things go south. First off, errors looking for the correct arch path (fixed by make ARCH=arm/mach-bcm2709), then next issues with asm (not asm-generic!) headers ... like for example, #include <asm/types.h>. This is (partially) fixed by ln -s /usr/include/arm-linux-gnueabihf/asm /home/pi/linux/include/asm => now I'm back to items missing that are in asm-generic (like, for example, asm/linkage.h). Not sure how to fix this one.
Has anyone been able to get this to work? Looking for any pointers you may have.