build: Add support for additional kernel config snippet

 * Add support for TARGET_KERNEL_ADDITIONAL_CONFIG which will append a
   config snippet from arch/$ARCH/configs/ to the main defconfig.
 * This can be used for various things, such as including DIAG support
   into debuggable builds.

Change-Id: Ifa48688a3f951dd8ecc9a13a27de3a476e7bf633

Author: Steve Kondik

core/tasks/kernel.mk

@@ -72,6 +72,9 @@ ifeq ($(KERNEL_ARCH),arm64)
   endif
 endif
 
+ifneq ($(TARGET_KERNEL_ADDITIONAL_CONFIG),)
+KERNEL_ADDITIONAL_CONFIG := $(TARGET_KERNEL_ADDITIONAL_CONFIG)
+endif
 
 ## Do be discontinued in a future version. Notify builder about target
 ## kernel format requirement
@@ -204,6 +207,10 @@ $(KERNEL_CONFIG): $(KERNEL_OUT)
 			echo "Overriding kernel config with '$(KERNEL_CONFIG_OVERRIDE)'"; \
 			echo $(KERNEL_CONFIG_OVERRIDE) >> $(KERNEL_OUT)/.config; \
 			$(MAKE) -C $(KERNEL_SRC) O=$(KERNEL_OUT) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) oldconfig; fi
+	$(hide) if [ ! -z "$(KERNEL_ADDITIONAL_CONFIG)" ]; then \
+			echo "Using additional config '$(KERNEL_ADDITIONAL_CONFIG)'"; \
+			cat $(KERNEL_SRC)/arch/$(KERNEL_ARCH)/configs/$(KERNEL_ADDITIONAL_CONFIG) >> $(KERNEL_OUT)/.config; \
+			$(MAKE) -C $(KERNEL_SRC) O=$(KERNEL_OUT) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) oldconfig; fi
 
 TARGET_KERNEL_BINARIES: $(KERNEL_OUT) $(KERNEL_CONFIG) $(KERNEL_HEADERS_INSTALL)
 	$(MAKE) $(MAKE_FLAGS) -C $(KERNEL_SRC) O=$(KERNEL_OUT) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) $(TARGET_PREBUILT_INT_KERNEL_TYPE)
@@ -232,6 +239,10 @@ $(KERNEL_HEADERS_INSTALL): $(KERNEL_OUT) $(KERNEL_CONFIG)
 			echo "Overriding kernel config with '$(KERNEL_CONFIG_OVERRIDE)'"; \
 			echo $(KERNEL_CONFIG_OVERRIDE) >> $(KERNEL_OUT)/.config; \
 			$(MAKE) -C $(KERNEL_SRC) O=$(KERNEL_OUT) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) oldconfig; fi
+	$(hide) if [ ! -z "$(KERNEL_ADDITIONAL_CONFIG)" ]; then \
+			echo "Using additional config '$(KERNEL_ADDITIONAL_CONFIG)'"; \
+			cat $(KERNEL_SRC)/arch/$(KERNEL_ARCH)/configs/$(KERNEL_ADDITIONAL_CONFIG) >> $(KERNEL_OUT)/.config; \
+			$(MAKE) -C $(KERNEL_SRC) O=$(KERNEL_OUT) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) oldconfig; fi
 
 kerneltags: $(KERNEL_OUT) $(KERNEL_CONFIG)
 	$(MAKE) -C $(KERNEL_SRC) O=$(KERNEL_OUT) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) tags