kbuild: Makefile.clean: make Kbuild and Makefile optional

AOSP commit b13ce9f4 "ARM64: add option to build
Image.gz/dtb combo" broke archclean / mrproper build
targets and we run into:
----------
./scripts/Makefile.clean:14: arch/arm64/boot/amd/Makefile: No such file or directory
make[2]: *** No rule to make target `arch/arm64/boot/amd/Makefile'.  Stop.
make[1]: *** [arch/arm64/boot/amd] Error 2
make: *** [archclean] Error 2
----------

This patch skip the missing Kbuild/Makefile reporting
error. It does the job (i.e cleanup dts/*/*.dtb and
do not spit out missing file error messages as well).

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>