see the following diagnostic from the build says the Linux Kernal headers could not be found

root@FriendlyELEC:~/rt8192cu# make dkms
rm -fr *.mod.c *.mod *.o .*.cmd *.ko *~
rm .tmp_versions -fr ; rm Module.symvers -fr
rm -fr Module.markers ; rm -fr modules.order
cd core/efuse ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
cd core ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
cd hal/rtl8192c/usb ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
cd hal/rtl8192c ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
cd hal ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
cd os_dep/linux ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
cd os_dep ; rm -fr *.mod.c *.mod *.o .*.cmd *.ko
rm -rf /usr/src/8192cu-4.0.29
mkdir /usr/src/8192cu-4.0.29 -p
cp . /usr/src/8192cu-4.0.29 -a
rm -rf /usr/src/8192cu-4.0.29/.git
dkms add -m 8192cu -v 4.0.29

Creating symlink /var/lib/dkms/8192cu/4.0.29/source ->
/usr/src/8192cu-4.0.29

DKMS: add completed.
dkms build -m 8192cu -v 4.0.29
Error! Your kernel headers for kernel 4.4.49-s5p6818 cannot be found.
Please install the linux-headers-4.4.49-s5p6818 package,
or use the --kernelsourcedir option to tell DKMS where it's located
Makefile:606: recipe for target 'dkms' failed
make: *** [dkms] Error 1

Then tried to install the kernal headers

root@FriendlyELEC:~# apt-cache search linux-headers-4.4.49-s5p6818
root@FriendlyELEC:~# apt-get install linux-headers-4.4.49-s5p6818
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package linux-headers-4.4.49-s5p6818
E: Couldn't find any package by glob 'linux-headers-4.4.49-s5p6818'

So how do I get the kernal headers for 4.4.49-s5p6818?