kbuild: refactor the check for missing config files

This commit refactors the check for missing configuration files, making
it easier to add more files to the list.

The format of the error message has been slightly changed, as follows:

[Before]

    ERROR: Kernel configuration is invalid.
           include/generated/autoconf.h or include/config/auto.conf are missing.
           Run 'make oldconfig && make prepare' on kernel src to fix it.

[After]

  ***
  ***  ERROR: Kernel configuration is invalid. The following files are missing:
  ***    - include/generated/autoconf.h
  ***    - include/config/auto.conf
  ***  Run "make oldconfig && make prepare" on kernel source to fix it.
  ***

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -781,17 +781,22 @@ $(KCONFIG_CONFIG):
 else # !may-sync-config
 # External modules and some install targets need include/generated/autoconf.h
 # and include/config/auto.conf but do not care if they are up-to-date.
-# Use auto.conf to trigger the test
+# Use auto.conf to show the error message
+
+checked-configs := include/generated/autoconf.h include/config/auto.conf
+missing-configs := $(filter-out $(wildcard $(checked-configs)), $(checked-configs))
+
+ifdef missing-configs
 PHONY += include/config/auto.conf
 
 include/config/auto.conf:
-	@test -e include/generated/autoconf.h -a -e $@ || (		\
-	echo >&2;							\
-	echo >&2 "  ERROR: Kernel configuration is invalid.";		\
-	echo >&2 "         include/generated/autoconf.h or $@ are missing.";\
-	echo >&2 "         Run 'make oldconfig && make prepare' on kernel src to fix it.";	\
-	echo >&2 ;							\
-	/bin/false)
+	@echo   >&2 '***'
+	@echo   >&2 '***  ERROR: Kernel configuration is invalid. The following files are missing:'
+	@printf >&2 '***    - %s\n' $(missing-configs)
+	@echo   >&2 '***  Run "make oldconfig && make prepare" on kernel source to fix it.'
+	@echo   >&2 '***'
+	@/bin/false
+endif
 
 endif # may-sync-config
 endif # need-config