scripts/Makefile.clean: Make Kbuild and Makefile optional

scripts/Makefile.clean: Make Kbuild and Makefile optional

During cleaning, some directories might not have a Makefile or Kbuild
file.  Previously scripts/Makefile.clean would spit out the following
error when neither file could be found:
ptyser@petert busybox $ make mrproper
scripts/Makefile.clean:17: /home/user/busybox/findutils/Makefile: No such file or directory
make[1]: *** No rule to make target `/home/user/busybox/findutils/Makefile'.  Stop.
make: *** [_clean_findutils] Error 2

This issue can be triggered by running 'make mrproper' in a freshly
cloned busybox repository.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>