scripts/kconfig/Makefile: Allow KBUILD_DEFCONFIG to be a target

Arch Makefiles can set KBUILD_DEFCONFIG to tell kbuild the name of the
defconfig that should be built by default.

However currently there is an assumption that KBUILD_DEFCONFIG points to
a file at arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG).

We would like to use a target, using merge_config, as our defconfig, so
adapt the logic in scripts/kconfig/Makefile to allow that.

To minimise the chance of breaking anything, we first check if
KBUILD_DEFCONFIG is a file, and if so we do the old logic. If it's not a
file, then we call the top-level Makefile with KBUILD_DEFCONFIG as the
target.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Michal Marek <mmarek@suse.com>

scripts/kconfig/Makefile

@@ -96,9 +96,12 @@ savedefconfig: $(obj)/conf
 defconfig: $(obj)/conf
 ifeq ($(KBUILD_DEFCONFIG),)
 	$< $(silent) --defconfig $(Kconfig)
-else
+else ifneq ($(wildcard arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)),)
 	@$(kecho) "*** Default configuration is based on '$(KBUILD_DEFCONFIG)'"
 	$(Q)$< $(silent) --defconfig=arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG) $(Kconfig)
+else
+	@$(kecho) "*** Default configuration is based on target '$(KBUILD_DEFCONFIG)'"
+	$(Q)$(MAKE) -f $(srctree)/Makefile $(KBUILD_DEFCONFIG)
 endif
 
 %_defconfig: $(obj)/conf