kbuild: unset variables in top Makefile instead of setting 0

There is no need to set 0 to variables such as config-targets,
mixed-targets, etc.

Unset instead of setting 0 in order to use 'ifdef' to test them.

I also renamed:

  config-targets  ->  config-build
  mixed-targets   ->  mixed-build
  dot-config      ->  need-config

to clarify what we are doing.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -272,32 +272,32 @@ no-dot-config-targets := $(clean-targets) \
 no-sync-config-targets := $(no-dot-config-targets) install %install \
 			   kernelrelease
 
-config-targets  := 0
-mixed-targets   := 0
-dot-config      := 1
+config-build	:=
+mixed-build	:=
+need-config	:= 1
 may-sync-config	:= 1
 
 ifneq ($(filter $(no-dot-config-targets), $(MAKECMDGOALS)),)
 	ifeq ($(filter-out $(no-dot-config-targets), $(MAKECMDGOALS)),)
-		dot-config := 0
+		need-config :=
 	endif
 endif
 
 ifneq ($(filter $(no-sync-config-targets), $(MAKECMDGOALS)),)
 	ifeq ($(filter-out $(no-sync-config-targets), $(MAKECMDGOALS)),)
-		may-sync-config := 0
+		may-sync-config :=
 	endif
 endif
 
 ifneq ($(KBUILD_EXTMOD),)
-	may-sync-config := 0
+	may-sync-config :=
 endif
 
 ifeq ($(KBUILD_EXTMOD),)
         ifneq ($(filter config %config,$(MAKECMDGOALS)),)
-                config-targets := 1
+		config-build := 1
                 ifneq ($(words $(MAKECMDGOALS)),1)
-                        mixed-targets := 1
+			mixed-build := 1
                 endif
         endif
 endif
@@ -305,18 +305,18 @@ endif
 # For "make -j clean all", "make -j mrproper defconfig all", etc.
 ifneq ($(filter $(clean-targets),$(MAKECMDGOALS)),)
         ifneq ($(filter-out $(clean-targets),$(MAKECMDGOALS)),)
-                mixed-targets := 1
+		mixed-build := 1
         endif
 endif
 
 # install and modules_install need also be processed one by one
 ifneq ($(filter install,$(MAKECMDGOALS)),)
         ifneq ($(filter modules_install,$(MAKECMDGOALS)),)
-	        mixed-targets := 1
+		mixed-build := 1
         endif
 endif
 
-ifeq ($(mixed-targets),1)
+ifdef mixed-build
 # ===========================================================================
 # We're called with mixed targets (*config and build targets).
 # Handle them one by one.
@@ -332,7 +332,7 @@ __build_one_by_one:
 		$(MAKE) -f $(srctree)/Makefile $$i; \
 	done
 
-else
+else # !mixed-build
 
 include scripts/Kbuild.include
 
@@ -544,7 +544,7 @@ endif
 # and from include/config/auto.conf.cmd to detect the compiler upgrade.
 CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1)
 
-ifeq ($(config-targets),1)
+ifdef config-build
 # ===========================================================================
 # *config targets only - make sure prerequisites are updated, and descend
 # in scripts/kconfig to make the *config target
@@ -561,7 +561,7 @@ config: scripts_basic outputmakefile FORCE
 %config: scripts_basic outputmakefile FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
 
-else
+else #!config-build
 # ===========================================================================
 # Build targets only - this includes vmlinux, arch specific targets, clean
 # targets and others. In general all targets except *config targets.
@@ -604,7 +604,7 @@ endif
 
 export KBUILD_MODULES KBUILD_BUILTIN
 
-ifeq ($(dot-config),1)
+ifdef need-config
 include include/config/auto.conf
 endif
 
@@ -652,8 +652,8 @@ ARCH_AFLAGS :=
 ARCH_CFLAGS :=
 include arch/$(SRCARCH)/Makefile
 
-ifeq ($(dot-config),1)
-ifeq ($(may-sync-config),1)
+ifdef need-config
+ifdef may-sync-config
 # Read in dependencies to all Kconfig* files, make sure to run syncconfig if
 # changes are detected. This should be included after arch/$(SRCARCH)/Makefile
 # because some architectures define CROSS_COMPILE there.
@@ -676,7 +676,7 @@ $(KCONFIG_CONFIG):
 # The syncconfig should be executed only once to make all the targets.
 %/auto.conf %/auto.conf.cmd %/tristate.conf: $(KCONFIG_CONFIG)
 	$(Q)$(MAKE) -f $(srctree)/Makefile syncconfig
-else
+else # !may-sync-config
 # External modules and some install targets need include/generated/autoconf.h
 # and include/config/auto.conf but do not care if they are up-to-date.
 # Use auto.conf to trigger the test
@@ -692,7 +692,7 @@ include/config/auto.conf:
 	/bin/false)
 
 endif # may-sync-config
-endif # $(dot-config)
+endif # need-config
 
 KBUILD_CFLAGS	+= $(call cc-option,-fno-delete-null-pointer-checks,)
 KBUILD_CFLAGS	+= $(call cc-disable-warning,frame-address,)
@@ -1809,8 +1809,8 @@ existing-targets := $(wildcard $(sort $(targets)))
 
 -include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
 
-endif   # ifeq ($(config-targets),1)
-endif   # ifeq ($(mixed-targets),1)
+endif # config-targets
+endif # mixed-build
 endif # need-sub-make
 
 PHONY += FORCE