kernel: prevent additional config from being always rebuilt

* A simple "make kernel" or incremental system build
   always triggers a complete kernel recompilation,
   even though the kernel sources were unchanged

 * The reason is because .additional_config is always
   considered new and invalidates the .config file,
   forcing the kernel to rebuild itself every time

 * Instead, prevent the /dev/null and KERNEL_OBJ dependencies
   having irrelevant timestamp changes with an empty variable

 * Command "make kernel" takes 3 seconds versus 40s before

Change-Id: I3b583077b7ef5873eebde3e4b0e790f5d7587ed3
Signed-off-by: Adrian DC <radian.dc@gmail.com>

build/tasks/kernel.mk

@@ -89,10 +89,13 @@ KERNEL_DEFCONFIG_SRC := $(KERNEL_DEFCONFIG_DIR)/$(KERNEL_DEFCONFIG)
 
 ifneq ($(TARGET_KERNEL_ADDITIONAL_CONFIG),)
 KERNEL_ADDITIONAL_CONFIG := $(TARGET_KERNEL_ADDITIONAL_CONFIG)
+KERNEL_ADDITIONAL_CONFIG_DEPENDENCY :=
 KERNEL_ADDITIONAL_CONFIG_SRC := $(KERNEL_DEFCONFIG_DIR)/$(KERNEL_ADDITIONAL_CONFIG)
     ifeq ("$(wildcard $(KERNEL_ADDITIONAL_CONFIG_SRC))","")
         $(warning TARGET_KERNEL_ADDITIONAL_CONFIG '$(TARGET_KERNEL_ADDITIONAL_CONFIG)' doesn't exist)
         KERNEL_ADDITIONAL_CONFIG_SRC := /dev/null
+    else
+        KERNEL_ADDITIONAL_CONFIG_DEPENDENCY := $(KERNEL_ADDITIONAL_CONFIG_SRC)
     endif
 else
     KERNEL_ADDITIONAL_CONFIG_SRC := /dev/null
@@ -239,10 +242,8 @@ define make-dtb-target
 $(call internal-make-kernel-target,$(DTB_OUT),$(1))
 endef
 
-$(KERNEL_OUT):
+$(KERNEL_ADDITIONAL_CONFIG_OUT): $(KERNEL_ADDITIONAL_CONFIG_DEPENDENCY)
 	mkdir -p $(KERNEL_OUT)
-
-$(KERNEL_ADDITIONAL_CONFIG_OUT): $(KERNEL_OUT)
 	$(hide) cmp -s $(KERNEL_ADDITIONAL_CONFIG_SRC) $@ || cp $(KERNEL_ADDITIONAL_CONFIG_SRC) $@;
 
 $(KERNEL_CONFIG): $(KERNEL_DEFCONFIG_SRC) $(KERNEL_ADDITIONAL_CONFIG_OUT)
@@ -288,13 +289,15 @@ kerneltags: $(KERNEL_CONFIG)
 
 .PHONY: kernelsavedefconfig alldefconfig
 
-kernelsavedefconfig: $(KERNEL_OUT)
+kernelsavedefconfig:
+	mkdir -p $(KERNEL_OUT)
 	$(call make-kernel-target,$(KERNEL_DEFCONFIG))
 	env KCONFIG_NOTIMESTAMP=true \
 		 $(call make-kernel-target,savedefconfig)
 	cp $(KERNEL_OUT)/defconfig $(KERNEL_DEFCONFIG_SRC)
 
-alldefconfig: $(KERNEL_OUT)
+alldefconfig:
+	mkdir -p $(KERNEL_OUT)
 	env KCONFIG_NOTIMESTAMP=true \
 		 $(call make-kernel-target,alldefconfig)