kbuild: allow architectures to specify defconfig file with KBUILD_DEFCONFIG

For some architectures is does not make sense to maintain a single default
configuration - arm is a good example here.
KBUILD_DEFCONFIG is introduced allowing relevant architectures to point out
a configuration kept in arch/$(ARCH)/configs as the configuration
to be used when executing 'make defconfig'.
This patch selects versatile_defconfig as the default configuration for arm.

The reason to keep defconfig is that is has proved valuable in many testing
scenarios when one are doing a cross architecture build of the kernel to
do a simple compile-time check of some changes.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -440,6 +440,12 @@ ifeq ($(config-targets),1)
 # *config targets only - make sure prerequisites are updated, and descend
 # in scripts/kconfig to make the *config target
 
+# Read arch specific Makefile to set KBUILD_DEFCONFIG as needed.
+# KBUILD_DEFCONFIG may point out an alternative default configuration
+# used for 'make defconfig'
+include $(srctree)/arch/$(ARCH)/Makefile
+export KBUILD_DEFCONFIG
+
 config: scripts_basic outputmakefile FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
 %config: scripts_basic outputmakefile FORCE

arch/arm/Makefile

@@ -13,6 +13,10 @@ OBJCOPYFLAGS	:=-O binary -R .note -R .comment -S
 GZFLAGS		:=-9
 #CFLAGS		+=-pipe
 
+# Do not use arch/arm/defconfig - it's always outdated.
+# Select a platform tht is kept up-to-date
+KBUILD_DEFCONFIG := versatile_defconfig
+
 ifeq ($(CONFIG_FRAME_POINTER),y)
 CFLAGS		+=-fno-omit-frame-pointer -mapcs -mno-sched-prolog
 endif

scripts/kconfig/Makefile

@@ -38,7 +38,12 @@ allmodconfig: $(obj)/conf
 	$< -m arch/$(ARCH)/Kconfig
 
 defconfig: $(obj)/conf
+ifeq ($(KBUILD_DEFCONFIG),)
 	$< -d arch/$(ARCH)/Kconfig
+else
+	@echo *** Default configuration is based on '$(KBUILD_DEFCONFIG)'
+	$(Q)$< -D arch/$(ARCH)/configs/$(KBUILD_DEFCONFIG) arch/$(ARCH)/Kconfig
+endif
 
 %_defconfig: $(obj)/conf
 	$(Q)$< -D arch/$(ARCH)/configs/$@ arch/$(ARCH)/Kconfig