speed up package prereq check (only run make on those directories that actually contain prereq checks)

git-svn-id: svn://svn.openwrt.org/openwrt/branches/buildroot-ng/openwrt@4927 3c298f89-4303-0410-b956-a3cf2f4a3e73

include/package.mk

@@ -163,13 +163,14 @@ define BuildPackage
     endif
 
     DUMPINFO += \
+	if [ "$$$$PREREQ_CHECK" = 1 ]; then echo "Prereq-Check: 1"; fi; \
 	echo "Version: $(VERSION)"; \
 	echo "Depends: $$(IDEPEND_$(1))"; \
 	echo "Build-Depends: $(PKG_BUILDDEP)"; \
 	echo "Category: $(CATEGORY)"; \
 	echo "Title: $(TITLE)"; \
 	echo "Description: $(DESCRIPTION)" | sed -e 's,\\,\n,g';
-
+	
     ifneq ($(URL),)
       DUMPINFO += \
 	echo; \

include/prereq.mk

@@ -22,6 +22,7 @@ prereq:
 .SILENT: $(TMP_DIR) prereq
 
 define Require
+  export PREREQ_CHECK=1
   ifeq ($$(CHECK_$(1)),)
     prereq: prereq-$(1)