imx6: fix generating bootfs in imagebuilder (FS#102)

The image builder does not bundle sub-directories within $(KERNEL_BUILD_DIR),
therfore the intermediate "ventana-uImage.boot" directory is not shipped,
leading to failures with "make image" later on.

To circumvent the issue, store the intermediate boot files as tar archive
instead of putting them into a directory to ensure that they end up in the
final image builder tarball. Fixes FS#102

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/imx6/image/Makefile

@@ -37,10 +37,16 @@ define Build/boot-overlay
 		--space-fixup --force-compr=zlib --squash-uids \
 		$(MKUBIFS_OPTS) -c 8124 \
 		-o $@.boot.ubifs -d $@.boot
+
+	$(TAR) -C $@.boot -cf $@.boot.tar .
 endef
 
 define Build/bootfs.tar.gz
-	$(TAR) -C $(IMAGE_KERNEL).boot \
+	rm -rf $@.boot
+	mkdir -p $@.boot
+
+	$(TAR) -C $@.boot -xf $(IMAGE_KERNEL).boot.tar
+	$(TAR) -C $@.boot \
 		--numeric-owner --owner=0 --group=0 --transform "s,./,./boot/," \
 		-czvf $@ .
 endef