ca-certificates: canonical the build dir

The previous build directory "build_dir/target-xx/work/"
contaminated the entire build directory.

Signed-off-by: Jianhui Zhao <[email protected]>

package/system/ca-certificates/Makefile

@@ -14,7 +14,6 @@ PKG_MAINTAINER:=
 PKG_SOURCE:=$(PKG_NAME)_$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=http://ftp.debian.org/debian/pool/main/c/ca-certificates
 PKG_HASH:=43766d5a436519503dfd65ab83488ae33ab4d4ca3d0993797b58c92eb9ed4e63
-PKG_BUILD_DIR:=$(BUILD_DIR)/work
 PKG_INSTALL:=1
 
 include $(INCLUDE_DIR)/package.mk
@@ -35,6 +34,13 @@ define Package/ca-bundle
   PROVIDES:=ca-certs
 endef
 
+define Build/Prepare
+	$(DECOMPRESS_CMD) $(HOST_TAR) -C $(PKG_BUILD_DIR) $(TAR_OPTIONS)
+	$(Build/Patch)
+endef
+
+MAKE_PATH := work
+
 define Build/Install
 	mkdir -p \
 		$(PKG_INSTALL_DIR)/usr/sbin \