scripts/package/Makefile: compare objtree with srctree instead of test KBUILD_OUTPUT

KBUILD_OUTPUT is always empty here, so it is useless to test it. But
while use O=.., objtree and srctree will be different. I compare them
instead.
Signed-off-by: Bin Wang <wbin00@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/package/Makefile

@@ -36,7 +36,7 @@ $(objtree)/kernel.spec: $(MKSPEC) $(srctree)/Makefile
 	$(CONFIG_SHELL) $(MKSPEC) > $@
 
 rpm-pkg rpm: $(objtree)/kernel.spec FORCE
-	@if test -n "$(KBUILD_OUTPUT)"; then \
+	@if test "$(objtree)" != "$(srctree)"; then \
 		echo "Building source + binary RPM is not possible outside the"; \
 		echo "kernel source tree. Don't set KBUILD_OUTPUT, or use the"; \
 		echo "binrpm-pkg target instead."; \