boot: Make fit_image_load() easier to maintain

Before fit_image_load() was created, The code to load kernels, ramdisks and devicetrees from a FIT was spread around many functions. By combining most of the code in one place, it became possible to add more features in a consistent way. The 'loadables' feature much easier to plumb in, for example.

While fit_image_load() was a substantial advance, it has never been a svelte function and the passing years have not been entirely kind. With a few new features on the horizon, this is a good time to improve the implementation.

This series splits much of the code from fit_image_load() into a number of smaller functions. Most of the changes are fairly mechanical, with just a few renames and tweaks here and there.

This should make the function much easier to maintain. It may also encourage someone to take a look at its callers, which could also use some attention.