I had the same problem, make dtbs not working when doing a little tweak for kernel sources (added a touchscreen driver).
My kernel was not even booting, and made the same mistake twice. Third time I opened editor and prepared to post my whining here,
but noticed where the mistake took place...
rm -rf linux
git clone --depth=1 https://github.com/raspberrypi/linux
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- bcm2709_defconfig
cp ../gt811_ts.c drivers/input/touchscreen
# edited drivers/input/touchscreen/Makefile and
make oldconfig # meep meep!!!
Not using ARCH directive makes oldconfig do strange things for the .config
After fixing this, make dtbs works again. HTH.