kbuild: use scripts/Makefile.clean

This commit refactors cleaning targets such as
clean, clobber, mrpropper, distclean
with scripts/Makefile.clean.

By using scripts/Makefile.clean, we can recursively descend
into subdirectories and delete generated files there.

We do not need add a big list of generated files
to the "clean" target.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index 686cb0d..5cd0f51 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -37,6 +37,10 @@
 
 subdir-ymn	:= $(addprefix $(obj)/,$(subdir-ymn))
 
+# Temporal work-around for U-Boot
+
+subdir-ymn	:= $(foreach f, $(subdir-ymn), $(if $(wildcard $f/Makefile),$f))
+
 # build a list of files to remove, usually relative to the current
 # directory