build: improve jobcontrol by adding "+" to called make

This fixes errors "warning: jobserver unavailable: using -j1.  Add '+' to parent make rule"
(Issue #596). This all seems to be caused by our make not passing the relavant "jobserver"
flags down to the sub-make we call. As of the missing flags the sub-makes fall back to -j1.
Some reading on jobserver can be found: http://make.mad-scientist.net/papers/jobserver-implementation/

Makefile

@@ -68,6 +68,7 @@ update-patches: .stamp-pre-patch .FORCE
 # clean up openwrt working copy
 openwrt-clean: stamp-clean-openwrt-cleaned .stamp-openwrt-cleaned
 .stamp-openwrt-cleaned: config.mk | $(OPENWRT_DIR) openwrt-clean-bin
+	+
 	cd $(OPENWRT_DIR); \
 	  ./scripts/feeds clean && \
 	  git clean -dff && git fetch && git reset --hard HEAD && \
@@ -81,7 +82,7 @@ openwrt-clean-bin:
 # update feeds
 feeds-update: stamp-clean-feeds-updated .stamp-feeds-updated
 .stamp-feeds-updated: .stamp-patched
-	@
+	@+
 	$(UMASK); GLUON_SITEDIR='$(GLUON_SITEDIR)' scripts/feeds.sh
 	touch $@
 
@@ -122,6 +123,7 @@ $(OPENWRT_DIR)/files: $(FW_DIR)/embedded-files
 
 # openwrt config
 $(OPENWRT_DIR)/.config: .stamp-feeds-updated $(TARGET_CONFIG) .stamp-build_rev $(OPENWRT_DIR)/dl
+	+
 	cat $(TARGET_CONFIG) >$(OPENWRT_DIR)/.config
 	# always replace CONFIG_VERSION_CODE by FW_REVISION
 	sed -i "/^CONFIG_VERSION_CODE=/c\CONFIG_VERSION_CODE=\"$(FW_REVISION)\"" $(OPENWRT_DIR)/.config
@@ -136,6 +138,7 @@ prepare: stamp-clean-prepared .stamp-prepared
 # compile
 compile: stamp-clean-compiled .stamp-compiled
 .stamp-compiled: .stamp-prepared openwrt-clean-bin
+	+
 	$(UMASK); \
 	  $(MAKE) -C $(OPENWRT_DIR) $(MAKE_ARGS)
 	touch $@
@@ -200,9 +203,11 @@ images: .stamp-images
 #                  prerequirement is a build OpenWRT
 ifeq ($(origin IB_FILE),command line)
 .stamp-images: $(FW_DIR)/embedded-files .FORCE
+	+
 	$(info IB_FILE explicitly defined; using it for building firmware-images)
 else
 .stamp-images: .stamp-compiled
+	+
 	$(info IB_FILE not defined; assuming called from inside regular build)
 	$(eval IB_FILE := $(shell ls -tr $(OPENWRT_DIR)/bin/targets/$(MAINTARGET)/$(SUBTARGET)/*-imagebuilder-*.tar.xz | tail -n1))
 endif
@@ -219,6 +224,7 @@ endif
 	touch $@
 
 setup-sdk: .stamp-patched
+	+
 	@if [ -z "$(SDK_FILE)" ]; then
 		echo Error: Please provide SDK-FILE by using "make SDK_FILE=<filename>"
 		exit 1

Makefile.autobuild

@@ -22,49 +22,49 @@ endif
 
 download: info .autobuild_stamp-download .FORCE
 .autobuild_stamp-download: $(OPENWRT_DIR)/.config
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs download
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs download
 	touch $@
 
 tools: .autobuild_stamp-download
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs tools/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs tools/install
 	touch .autobuild_stamp-$@
 
 toolchain: 
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs toolchain/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs toolchain/install
 	touch .autobuild_stamp-$@
 
 kmods: 
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/compile
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/compile
 	touch .autobuild_stamp-$@
 
 packages: .autobuild_stamp-packages .FORCE
 .autobuild_stamp-packages:
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs package/compile
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs package/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs package/compile
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs package/install
 	touch $@
 
 images: .autobuild_stamp-packages
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/install
 	touch .autobuild_stamp-$@
 
 linux:
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/linux/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/linux/install
 	touch .autobuild_stamp-$@
 
 imagebuilder:
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/imagebuilder/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/imagebuilder/install
 	touch .autobuild_stamp-$@
 
 sdk:
-	$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/sdk/install
+	+$(MAKE) -C $(OPENWRT_DIR) BUILD_LOG_DIR=$(FW_DIR)/logs target/sdk/install
 	touch .autobuild_stamp-$@
 
 travis: $(OPENWRT_DIR)/.config
-	travis_wait $(MAKE) -C $(OPENWRT_DIR) toolchain/install
+	+travis_wait $(MAKE) -C $(OPENWRT_DIR) toolchain/install
 
 buildbot: $(OPENWRT_DIR)/.config
-	$(MAKE) compile
-	$(MAKE) version-file
+	+$(MAKE) compile
+	+$(MAKE) version-file
 	# copy imagebuilder, sdk and toolchain (if existing)
 	# remove old versions
 	rm -rf $(FW_TARGET_DIR)
@@ -81,13 +81,13 @@ buildbot: $(OPENWRT_DIR)/.config
 	cp -a $(OPENWRT_DIR)/bin/packages $$PACKAGES_DIR/
 	rm -rf $(OPENWRT_DIR)
 	rm -rf $(FW_DIR)/dl
-	$(MAKE) IB_FILE=$(FW_TARGET_DIR)/*-imagebuilder-*.tar.xz images
+	+$(MAKE) IB_FILE=$(FW_TARGET_DIR)/*-imagebuilder-*.tar.xz images
 
 config: info $(OPENWRT_DIR)/.config
 $(OPENWRT_DIR)/.config: $(FW_DIR)/configs/common-autobuild.config
 	TARGET_CONFIG_AUTOBUILD=$(FW_DIR)/configs/common-autobuild.config
 	cat $(FW_DIR)/configs/common-autobuild.config >>$(FW_DIR)/configs/common.config
-	$(MAKE) prepare
+	+$(MAKE) prepare
 
 next-buildstep-for-%:
 #	@echo this step: $*