kbuild: fix build with O=..

.kernelrelease was saved in same directory as kernel source also
with make O=...
Make sure we kick in the normal logic to shift to the output directory
when we build .kernelrelease after executing *config.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---

Makefile

@@ -442,7 +442,7 @@ export KBUILD_DEFCONFIG
 config %config: scripts_basic outputmakefile FORCE
 	$(Q)mkdir -p include/linux
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
-	$(Q)$(MAKE) .kernelrelease
+	$(Q)$(MAKE) -C $(srctree) KBUILD_SRC= .kernelrelease
 
 else
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