spl: fit: add support for post-processing of images

spl: fit: add support for post-processing of images

The next stage boot loader image and the selected FDT can be post-
processed by board/platform/device-specific code, which can include
modifying the size and altering the starting source address before
copying these binary blobs to their final destination. This might be
desired to do things like strip headers or footers attached to the
images before they were packaged into the FIT, or to perform operations
such as decryption or authentication. Introduce new configuration
option CONFIG_SPL_FIT_IMAGE_POST_PROCESS to allow controlling this
feature. If enabled, a platform-specific post-process function must
be provided.

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