kernel: Add support for external kernel configs

For targets that uses mainline kernel, the kernel tree usually only
contains configs on top of the upstream repository.

In such case, if we put the configs in elsewhere, we can simply just
track the upstream kernel repo, without needing to import configs.

Example usage:

TARGET_KERNEL_CONFIG_EXT := \
    $(COMMON_PATH)/kconfig/platform_defconfig \
    $(COMMON_PATH)/kconfig/common-addons.config

TARGET_KERNEL_CONFIG_EXT += \
    $(DEVICE_PATH)/kconfig/device.config

Change-Id: I021ffb76d0c0612343cff7b5c81541c2b88ef7e6

Author: me-cafebabe

build/tasks/kernel.mk

@@ -26,6 +26,10 @@
 #                                          while all the others are fragments that will be merged
 #                                          to main one in .config.
 #   TARGET_KERNEL_RECOVERY_CONFIG      = Same as above, but applicable to recovery kernel instead.
+#   TARGET_KERNEL_CONFIG_EXT           = List of path to external kernel defconfigs.
+#                                          Same as TARGET_KERNEL_CONFIG, but paths are specified
+#                                          instead of filenames.
+#   TARGET_KERNEL_RECOVERY_CONFIG_EXT  = Same as above, but applicable to recovery kernel instead.
 #   TARGET_KERNEL_VARIANT_CONFIG       = Variant defconfig, optional
 #   TARGET_KERNEL_SELINUX_CONFIG       = SELinux defconfig, optional
 #
@@ -62,6 +66,11 @@
 #
 #   KERNEL_LTO                         = Optional, force LTO to none / thin / full
 #
+#   MERGE_ALL_KERNEL_CONFIGS_AT_ONCE   = Optional, whether or not to merge all kernel config
+#                                          fragments in one merge_configs.sh call. if true,
+#                                          kernel config fragments will get merged faster, but
+#                                          may cause some differences.
+#
 #   NEED_KERNEL_MODULE_ROOT            = Optional, if true, install kernel
 #                                          modules in root instead of vendor
 #   NEED_KERNEL_MODULE_SYSTEM          = Optional, if true, install kernel
@@ -81,8 +90,11 @@ ifneq ($(TARGET_NO_KERNEL_OVERRIDE),true)
 ## Externally influenced variables
 KERNEL_SRC := $(TARGET_KERNEL_SOURCE)
 # kernel configuration - mandatory
+MERGE_ALL_KERNEL_CONFIGS_AT_ONCE ?= false
 KERNEL_DEFCONFIG := $(TARGET_KERNEL_CONFIG)
+KERNEL_DEFCONFIG_EXT := $(TARGET_KERNEL_CONFIG_EXT)
 RECOVERY_DEFCONFIG := $(TARGET_KERNEL_RECOVERY_CONFIG)
+RECOVERY_DEFCONFIG_EXT := $(TARGET_KERNEL_RECOVERY_CONFIG_EXT)
 VARIANT_DEFCONFIG := $(TARGET_KERNEL_VARIANT_CONFIG)
 SELINUX_DEFCONFIG := $(TARGET_KERNEL_SELINUX_CONFIG)
 # dtb generation - optional
@@ -112,12 +124,13 @@ else
 KERNEL_DEFCONFIG_ARCH := $(KERNEL_ARCH)
 endif
 KERNEL_DEFCONFIG_DIR := $(KERNEL_SRC)/arch/$(KERNEL_DEFCONFIG_ARCH)/configs
+
 ALL_KERNEL_DEFCONFIG_SRCS := $(foreach config,$(KERNEL_DEFCONFIG),$(KERNEL_DEFCONFIG_DIR)/$(config))
+ALL_KERNEL_DEFCONFIG_SRCS += $(KERNEL_DEFCONFIG_EXT)
 ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS := $(foreach config,$(RECOVERY_DEFCONFIG),$(KERNEL_DEFCONFIG_DIR)/$(config))
+ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS += $(RECOVERY_DEFCONFIG_EXT)
 
-BASE_KERNEL_DEFCONFIG := $(word 1, $(KERNEL_DEFCONFIG))
 BASE_KERNEL_DEFCONFIG_SRC := $(word 1, $(ALL_KERNEL_DEFCONFIG_SRCS))
-BASE_RECOVERY_KERNEL_DEFCONFIG := $(word 1, $(RECOVERY_DEFCONFIG))
 BASE_RECOVERY_KERNEL_DEFCONFIG_SRC := $(word 1, $(ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS))
 
 ifeq ($(TARGET_PREBUILT_KERNEL),)
@@ -168,7 +181,7 @@ ifeq "$(wildcard $(KERNEL_SRC) )" ""
     endif
 else
     NEEDS_KERNEL_COPY := true
-    ifeq ($(TARGET_KERNEL_CONFIG),)
+    ifeq ($(TARGET_KERNEL_CONFIG)$(TARGET_KERNEL_CONFIG_EXT),)
         $(warning **********************************************************)
         $(warning * Kernel source found, but no configuration was defined  *)
         $(warning * Please add the TARGET_KERNEL_CONFIG variable to your   *)
@@ -298,9 +311,16 @@ endef
 
 # Generate kernel .config from a given defconfig
 # $(1): Output path (The value passed to O=)
-# $(2): The defconfig to process (just the filename, no need for full path to file)
+# $(2): The defconfig to process (full path to file)
 define make-kernel-config
-	$(call internal-make-kernel-target,$(1),VARIANT_DEFCONFIG=$(VARIANT_DEFCONFIG) SELINUX_DEFCONFIG=$(SELINUX_DEFCONFIG) $(2))
+	cp $(word 1,$(2)) $(1)/.config
+	$(call internal-make-kernel-target,$(1),olddefconfig)
+	$(if $(filter true,$(MERGE_ALL_KERNEL_CONFIGS_AT_ONCE)),\
+		$(KERNEL_SRC)/scripts/kconfig/merge_config.sh -m -O $(1) $(1)/.config $(filter %.config,$(2)),
+		$(foreach config,$(filter %.config,$(2)),
+			$(KERNEL_SRC)/scripts/kconfig/merge_config.sh -m -O $(1) $(1)/.config $(config); \
+			$(call internal-make-kernel-target,$(1),olddefconfig);))
+	$(call internal-make-kernel-target,$(1),VARIANT_DEFCONFIG=$(VARIANT_DEFCONFIG) SELINUX_DEFCONFIG=$(SELINUX_DEFCONFIG) olddefconfig)
 	$(hide) if [ "$(KERNEL_LTO)" = "none" ]; then \
 			$(KERNEL_SRC)/scripts/config --file $(1)/.config \
 			-d LTO_CLANG \
@@ -485,7 +505,7 @@ $(KERNEL_OUT):
 
 $(KERNEL_CONFIG): $(KERNEL_OUT) $(ALL_KERNEL_DEFCONFIG_SRCS)
 	@echo "Building Kernel Config"
-	$(call make-kernel-config,$(KERNEL_OUT),$(KERNEL_DEFCONFIG))
+	$(call make-kernel-config,$(KERNEL_OUT),$(ALL_KERNEL_DEFCONFIG_SRCS))
 
 $(TARGET_PREBUILT_INT_KERNEL): $(KERNEL_CONFIG) $(DEPMOD) $(DTC) $(KERNEL_MODULES_PARTITION_FILE_LIST) $(SYSTEM_KERNEL_MODULES_PARTITION_FILE_LIST)
 	@echo "Building Kernel Image ($(BOARD_KERNEL_IMAGE_NAME))"
@@ -567,7 +587,7 @@ kerneltags: $(KERNEL_CONFIG)
 .PHONY: kernelsavedefconfig alldefconfig kernelconfig recoverykernelconfig
 
 kernelsavedefconfig: $(KERNEL_OUT)
-	$(call make-kernel-config,$(KERNEL_OUT),$(BASE_KERNEL_DEFCONFIG))
+	$(call make-kernel-config,$(KERNEL_OUT),$(BASE_KERNEL_DEFCONFIG_SRC))
 	$(call make-kernel-target,savedefconfig)
 	cp $(KERNEL_OUT)/defconfig $(BASE_KERNEL_DEFCONFIG_SRC)
 
@@ -577,11 +597,11 @@ alldefconfig: $(KERNEL_OUT)
 
 kernelconfig: $(KERNEL_OUT) $(ALL_KERNEL_DEFCONFIG_SRCS)
 	@echo "Building Kernel Config"
-	$(call make-kernel-config,$(KERNEL_OUT),$(KERNEL_DEFCONFIG))
+	$(call make-kernel-config,$(KERNEL_OUT),$(ALL_KERNEL_DEFCONFIG_SRCS))
 
 recoverykernelconfig: $(KERNEL_OUT) $(ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS)
 	@echo "Building Recovery Kernel Config"
-	$(call make-kernel-config,$(RECOVERY_KERNEL_OUT),$(RECOVERY_DEFCONFIG))
+	$(call make-kernel-config,$(RECOVERY_KERNEL_OUT),$(ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS))
 
 ifeq (true,$(filter true, $(TARGET_NEEDS_DTBOIMAGE) $(BOARD_KERNEL_SEPARATED_DTBO)))
 ifneq ($(BOARD_CUSTOM_DTBOIMG_MK),)
@@ -680,7 +700,7 @@ $(RECOVERY_KERNEL_OUT):
 
 $(RECOVERY_KERNEL_CONFIG): $(ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS)
 	@echo "Building Recovery Kernel Config"
-	$(call make-kernel-config,$(RECOVERY_KERNEL_OUT),$(RECOVERY_DEFCONFIG))
+	$(call make-kernel-config,$(RECOVERY_KERNEL_OUT),$(ALL_RECOVERY_KERNEL_DEFCONFIG_SRCS))
 
 $(TARGET_PREBUILT_INT_RECOVERY_KERNEL): $(RECOVERY_KERNEL_CONFIG) $(DEPMOD) $(DTC)
 	@echo "Building Recovery Kernel Image ($(BOARD_KERNEL_IMAGE_NAME))"