build: add diffconfig target

Add a "diffconfig" build target which stores the output of
"scripts/diffconfig.sh" as "config.seed" in the image output directory and
invoke that target by default.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

Makefile

@@ -87,9 +87,13 @@ prereq: $(target/stamp-prereq) tmp/.prereq_packages
 checksum: FORCE
 	$(call sha256sums,$(BIN_DIR))
 
+diffconfig: FORCE
+	$(SCRIPT_DIR)/diffconfig.sh > $(BIN_DIR)/config.seed
+
 prepare: .config $(tools/stamp-install) $(toolchain/stamp-install)
 world: prepare $(target/stamp-compile) $(package/stamp-compile) $(package/stamp-install) $(target/stamp-install) FORCE
 	$(_SINGLE)$(SUBMAKE) -r package/index
+	$(_SINGLE)$(SUBMAKE) -r diffconfig
 	$(_SINGLE)$(SUBMAKE) -r checksum
 
 .PHONY: clean dirclean prereq prepare world package/symlinks package/symlinks-install package/symlinks-clean