build: make sure build fails when running kconfig fails

build: make sure build fails when running kconfig fails

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>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>