kbuild: external modules shall not check config consistency

external modules needs include/linux/autoconf.h and include/config/auto.conf
but skip the integrity test of these. Even with a newer Kconfig file we
shall just proceed since external modules simply uses the kernel source and
shall not attempt to modify it.
Error out if a config fiel is missing since they are mandatory.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -436,12 +436,13 @@ core-y		:= usr/
 endif # KBUILD_EXTMOD
 
 ifeq ($(dot-config),1)
-# In this section, we need .config
+# Read in config
+-include include/config/auto.conf
 
+ifeq ($(KBUILD_EXTMOD),)
 # Read in dependencies to all Kconfig* files, make sure to run
 # oldconfig if changes are detected.
 -include include/config/auto.conf.cmd
--include include/config/auto.conf
 
 # To avoid any implicit rule to kick in, define an empty command
 $(KCONFIG_CONFIG) include/config/auto.conf.cmd: ;
@@ -451,16 +452,27 @@ $(KCONFIG_CONFIG) include/config/auto.conf.cmd: ;
 # if auto.conf.cmd is missing then we are probably in a cleaned tree so
 # we execute the config step to be sure to catch updated Kconfig files
 include/config/auto.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
-ifeq ($(KBUILD_EXTMOD),)
 	$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
 else
-	$(error kernel configuration not valid - run 'make prepare' in $(srctree) to update it)
-endif
+# external modules needs include/linux/autoconf.h and include/config/auto.conf
+# but do not care if they are up-to-date. Use auto.conf to trigger the test
+PHONY += include/config/auto.conf
+
+include/config/auto.conf:
+	$(Q)test -e include/linux/autoconf.h -a -e $@ || (		\
+	echo;								\
+	echo "  ERROR: Kernel configuration is invalid.";		\
+	echo "         include/linux/autoconf.h or $@ are missing.";	\
+	echo "         Run 'make oldconfig && make prepare' on kernel src to fix it.";	\
+	echo;								\
+	/bin/false)
+
+endif # KBUILD_EXTMOD
 
 else
 # Dummy target needed, because used as prerequisite
 include/config/auto.conf: ;
-endif
+endif # $(dot-config)
 
 # The all: target is the default when no target is given on the
 # command line.