spl: fit: Break out some functions into a common file

Some of the functions within spl_fit will be used for non spl purposes.
Instead of duplicating functions simply break the functions to be reused
into its own file.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Only add the new define to image.h, otherwise we see breakage
due to massive include leakage into host tools in some cases]
Signed-off-by: Tom Rini <trini@konsulko.com>