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>