[source] build: add diffconfig target

[LEDE Commits]

jow pushed a commit to source.git, branch master:
https://git.lede-project.org/2a72a916ab24b7c844886444d741fcb5d6191596

commit 2a72a916ab24b7c844886444d741fcb5d6191596
Author: Jo-Philipp Wich <jo at mein.io>
AuthorDate: Wed Jan 4 09:03:50 2017 +0100

    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 at mein.io>
---
 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index 51ed5a1..3450f7e 100644
--- a/Makefile
+++ b/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