Loading a FIT is useful for other VBE methods, such as ABrec. Create a
new function to handling reading it.
Signed-off-by: Simon Glass <sjg@chromium.org>
Move a few things into a new, common header file so that vbe-simple can
share code with the upcoming abrec.
Put struct simple_nvdata in it and rename it.
Signed-off-by: Simon Glass <sjg@chromium.org>