linux-phytec-ti

kbuild: Improve DT overlay building and cleaning

With this patch dtbo building is going to be working for
arch/arm64/boot/dts/ti/ (which is not working atm).

It is also going to fix the 'make clean' and removes all created dtbo files
as well.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>