arm: omap-common: Update to generate secure U-Boot FIT blob

arm: omap-common: Update to generate secure U-Boot FIT blob

Adds commands so that when a secure device is in use and the SPL is
built to load a FIT image (with combined U-Boot binary and various
DTBs), these components that get fed into the FIT are all processed to
be signed/encrypted/etc. as per the operations performed by the
secure-binary-image.sh script of the TI SECDEV package. Furthermore,
perform minor comments cleanup to make better use of the available
space.

Signed-off-by: Daniel Allred <d-allred@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>