common: image: Add support for post-processing of images

This commit allows injecting a board/platform/device-specific post-
processing function into the FIT image data loading process, which can
include modifying the size and altering the starting source address of
an image data artifact. 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_FIT_IMAGE_POST_PROCESS to
allow controlling this feature. If enabled, a platform-specific post-
process function must be provided.

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