spl: record boot_device into spl_image and call spl_perform_fixups

spl: record boot_device into spl_image and call spl_perform_fixups

On some boards, we want to give the board/architecture-specific code a
chance to look at where the next image has been loaded from and
perform fixups before starting the next image.  This is of particular
importance, when we probe multiple devices for bootable payloads and
boot the first one found.

This change adds the following:
 - we record the boot_device used into the spl_image structure
 - we provide an extension-point for boards/architectures that can
   perform late fixups depending on a fully populated spl_image
   structure (i.e. we'll know the final boot_device and have info
   on the image type and operating system to be booted).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>