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>
diff --git a/Makefile b/Makefile
index f1093b9..697eaf6 100644
--- a/Makefile
+++ b/Makefile
@@ -393,6 +393,11 @@
        SRCARCH := sh
 endif
 
+export cross_compiling :=
+ifneq ($(SRCARCH),$(SUBARCH))
+cross_compiling := 1
+endif
+
 KCONFIG_CONFIG	?= .config
 export KCONFIG_CONFIG