[PATCH 2/2] assemble_image: Copy a UBIFS image to the filesystem for SD Card image builds

* For SD Card image builds, also make a ubi image and copy it to /boot in the root filesystem
  This is used by the flashing script to ubiformat the NAND

Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>

The ubifs addition needs to be opt-in, not forcefully enabled for all builds