kbuild: Do not overwrite include/config/kernel.release needlessly

Use filechk to detect if the content changed or not.
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -794,10 +794,13 @@ PHONY += $(vmlinux-dirs)
 $(vmlinux-dirs): prepare scripts
 	$(Q)$(MAKE) $(build)=$@
 
+define filechk_kernel.release
+	echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
+endef
+
 # Store (new) KERNELRELEASE string in include/config/kernel.release
 include/config/kernel.release: include/config/auto.conf FORCE
-	$(Q)rm -f $@
-	$(Q)echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))" > $@
+	$(call filechk,kernel.release)
 
 
 # Things we need to do before we recursively start building the kernel