mkimage: Put FIT loading in function and tidy error handling

The fit_handle_file() function is quite long - split out the part that
loads and checks a FIT into its own function. We will use this
function for storing public keys into a destination FDT file.

The error handling is currently a bit repetitive - tidy it.

Signed-off-by: Simon Glass <sjg@chromium.org>
1 file changed