kbuild: include config.mk when auto.conf is not older than .config

Since the Kconfig conversion, config.mk has been included only when
include/config/auto.conf is newer than the .config file.

It causes build error if both files have the same time-stamps.
It is actually possible because EXT* file systems have a 1s time-stamp
resolution.

The config.mk should be included when include/config/auto.conf is
*not older* than the .config file.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Tom Rini <trini@konsulko.com>
Reported-by: York Sun <yorksun@freescale.com>
Reported-by: Stephen Warren <swarren@nvidia.com>
Reported-by: Matthew Gerlach <mgerlach@opensource.altera.com>
Tested-by: Stephen Warren <swarren@nvidia.com>

Makefile

@@ -513,12 +513,16 @@ include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
 # is up-to-date. When we switch to a different board configuration, old CONFIG
 # macros are still remaining in include/config/auto.conf. Without the following
 # gimmick, wrong config.mk would be included leading nasty warnings/errors.
-autoconf_is_current := $(if $(wildcard $(KCONFIG_CONFIG)),$(shell find . \
-		-path ./include/config/auto.conf -newer $(KCONFIG_CONFIG)))
-ifneq ($(autoconf_is_current),)
+ifneq ($(wildcard $(KCONFIG_CONFIG)),)
+ifneq ($(wildcard include/config/auto.conf),)
+autoconf_is_old := $(shell find . -path ./$(KCONFIG_CONFIG) -newer \
+						include/config/auto.conf)
+ifeq ($(autoconf_is_old),)
 include $(srctree)/config.mk
 include $(srctree)/arch/$(ARCH)/Makefile
 endif
+endif
+endif
 
 # If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use
 # that (or fail if absent).  Otherwise, search for a linker script in a