arago-tisdk: Implement EXTRA_TISDK_PREBUILT images.

This set of files from the deploy/ directory will be copied onto
the board-support/prebuilt-images/ directory on the SDK.

Signed-off-by: Daniel Diaz <ddiaz@ti.com>
arago-tisdk: Ship prebuilt versions of UBL (ARM and DSP).

Signed-off-by: Daniel Diaz <ddiaz@ti.com>