kernel: Implement build time LTO choice

Change-Id: I898eb50fadb6bde5beebf46f443c5de7e6acc907

build/tasks/kernel.mk

@@ -55,6 +55,8 @@
 #                                                      aarch64-linux-gnu- for arm64
 #                                                      x86_64-linux-gnu- for x86
 #
+#   KERNEL_LTO                         = Optional, force LTO to none / thin / full
+#
 #   NEED_KERNEL_MODULE_ROOT            = Optional, if true, install kernel
 #                                          modules in root instead of vendor
 #   NEED_KERNEL_MODULE_SYSTEM          = Optional, if true, install kernel
@@ -275,6 +277,31 @@ endef
 # $(2): The defconfig to process (just the filename, no need for full path to file)
 define make-kernel-config
 	$(call internal-make-kernel-target,$(1),VARIANT_DEFCONFIG=$(VARIANT_DEFCONFIG) SELINUX_DEFCONFIG=$(SELINUX_DEFCONFIG) $(2))
+	$(hide) if [ "$(KERNEL_LTO)" = "none" ]; then \
+			$(KERNEL_SRC)/scripts/config --file $(1)/.config \
+			-d LTO_CLANG \
+			-e LTO_NONE \
+			-d LTO_CLANG_THIN \
+			-d LTO_CLANG_FULL \
+			-d THINLTO; \
+			$(call make-kernel-target,olddefconfig); \
+		elif [ "$(KERNEL_LTO)" = "thin" ]; then \
+			$(KERNEL_SRC)/scripts/config --file $(1)/.config \
+			-e LTO_CLANG \
+			-d LTO_NONE \
+			-e LTO_CLANG_THIN \
+			-d LTO_CLANG_FULL \
+			-e THINLTO; \
+			$(call make-kernel-target,olddefconfig); \
+		elif [ "$(KERNEL_LTO)" = "full" ]; then \
+			$(KERNEL_SRC)/scripts/config --file $(1)/.config \
+			-e LTO_CLANG \
+			-d LTO_NONE \
+			-d LTO_CLANG_THIN \
+			-e LTO_CLANG_FULL \
+			-d THINLTO; \
+			$(call make-kernel-target,olddefconfig); \
+		fi
 	$(hide) if [ ! -z "$(KERNEL_CONFIG_OVERRIDE)" ]; then \
 			echo "Overriding kernel config with '$(KERNEL_CONFIG_OVERRIDE)'"; \
 			echo $(KERNEL_CONFIG_OVERRIDE) >> $(1)/.config; \