kbuild: reduce KBUILD_SINGLE_TARGETS as descending into subdirectories

KBUILD_SINGLE_TARGETS does not need to contain all the targets.
Change it to keep track the targets only from the current directory
and its subdirectories.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.build

@@ -469,15 +469,15 @@ targets += $(call intermediate_targets, .asn1.o, .asn1.c .asn1.h) \
 
 ifdef single-build
 
+KBUILD_SINGLE_TARGETS := $(filter $(obj)/%, $(KBUILD_SINGLE_TARGETS))
+
 curdir-single := $(sort $(foreach x, $(KBUILD_SINGLE_TARGETS), \
 			$(if $(filter $(x) $(basename $(x)).o, $(targets)), $(x))))
 
 # Handle single targets without any rule: show "Nothing to be done for ..." or
 # "No rule to make target ..." depending on whether the target exists.
 unknown-single := $(filter-out $(addsuffix /%, $(subdir-ym)), \
-			$(filter $(obj)/%, \
-				$(filter-out $(curdir-single), \
-					$(KBUILD_SINGLE_TARGETS))))
+			$(filter-out $(curdir-single), $(KBUILD_SINGLE_TARGETS)))
 
 __build: $(curdir-single) $(subdir-ym)
 ifneq ($(unknown-single),)