fs/iso9660: don't pollute $(BUILD_DIR) with temp dir

Use the newly-introdued $(FS_DIR) location to store temporary files.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

fs/iso9660/iso9660.mk

@@ -35,7 +35,7 @@ ROOTFS_ISO9660_USE_INITRD = YES
 endif
 
 ifeq ($(ROOTFS_ISO9660_USE_INITRD),YES)
-ROOTFS_ISO9660_TARGET_DIR = $(BUILD_DIR)/rootfs.iso9660.tmp
+ROOTFS_ISO9660_TARGET_DIR = $(FS_DIR)/rootfs.iso9660.tmp
 define ROOTFS_ISO9660_CREATE_TEMPDIR
 	$(RM) -rf $(ROOTFS_ISO9660_TARGET_DIR)
 	mkdir -p $(ROOTFS_ISO9660_TARGET_DIR)