kconfig: use /boot/config-* etc. as DEFCONFIG_LIST only for native build

When the .config file is missing, 'make config', 'make menuconfig', etc.
uses a file listed in DEFCONFIG_LIST, if found, as base configuration.

Ususally, /boot/config-$(uname -r) exists, and is used as default.

However, when you are cross-compiling the kernel, it does not make
sense to use /boot/config-* on the build host. It should default to
arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG).

UML previously did not use DEFCONFIG_LIST at all, but it should be
able to use arch/um/configs/$(KBUILD_DEFCONFIG) as a base config file.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -393,6 +393,11 @@ ifeq ($(ARCH),sh64)
        SRCARCH := sh
 endif
 
+export cross_compiling :=
+ifneq ($(SRCARCH),$(SUBARCH))
+cross_compiling := 1
+endif
+
 KCONFIG_CONFIG	?= .config
 export KCONFIG_CONFIG
 

scripts/kconfig/Makefile

@@ -18,14 +18,14 @@ silent := -s
 endif
 
 export KCONFIG_DEFCONFIG_LIST :=
-ifneq ($(SRCARCH),um)
+ifndef cross_compiling
 kernel-release := $(shell uname -r)
-KCONFIG_DEFCONFIG_LIST := \
+KCONFIG_DEFCONFIG_LIST += \
 	/lib/modules/$(kernel-release)/.config \
 	/etc/kernel-config \
-	/boot/config-$(kernel-release) \
-	arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)
+	/boot/config-$(kernel-release)
 endif
+KCONFIG_DEFCONFIG_LIST += arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)
 
 # We need this, in case the user has it in its environment
 unexport CONFIG_