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

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 <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
[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 <[email protected]>