[PATCH] build: make sure build fails when running kconfig fails

[Jan Beulich <jbeulich@suse.com>]

Because of using "-include", failure to (re)build auto.conf (with
auto.conf.cmd produced as a secondary target) won't stop make from
continuing the build. Arrange for it being possible to drop the - from
Rules.mk, requiring that the include be skipped for tools-only targets.
Note that relying on the inclusion in those cases wouldn't be correct
anyway, as it might be a stale file (yet to be rebuilt) which would be
included, while during initial build, the file would be absent
altogether.

Fixes: 8d4c17a90b0a ("xen/build: silence make warnings about missing auto.conf*")
Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
An alternative to the %.conf rule adjustment would be to remove the file
only when the sub-make there failed.

--- a/xen/Makefile
+++ b/xen/Makefile
@@ -375,6 +375,7 @@ $(KCONFIG_CONFIG): tools_fixdep
 # This exploits the 'multi-target pattern rule' trick.
 # The syncconfig should be executed only once to make all the targets.
 include/config/%.conf include/config/%.conf.cmd: $(KCONFIG_CONFIG)
+	$(Q)rm -f include/config/$*.conf
 	$(Q)$(MAKE) $(build)=tools/kconfig syncconfig
 
 ifeq ($(CONFIG_DEBUG),y)
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -15,7 +15,9 @@ srcdir := $(srctree)/$(src)
 PHONY := __build
 __build:
 
--include $(objtree)/include/config/auto.conf
+ifneq ($(firstword $(subst /, ,$(obj))),tools)
+include $(objtree)/include/config/auto.conf
+endif
 
 include $(XEN_ROOT)/Config.mk
 include $(srctree)/scripts/Kbuild.include