kbuild: fix bugs in cleaning targets

"make clean", "make clobber", "make mrproper" and "make distclean"
missed to clean-up some files when they were run with
O=<some_dir> option.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Wolfgang Denk <wd@denx.de>
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index 5cd0f51..d6dcd47 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -39,7 +39,8 @@
 
 # Temporal work-around for U-Boot
 
-subdir-ymn	:= $(foreach f, $(subdir-ymn), $(if $(wildcard $f/Makefile),$f))
+subdir-ymn	:= $(foreach f, $(subdir-ymn), \
+				$(if $(wildcard $(srctree)/$f/Makefile),$f))
 
 # build a list of files to remove, usually relative to the current
 # directory