doc: mkimage: Further document -o and -R

Despite the original description of these options, they are not always
image names, or even files. Some image types use these options to convey
configuration directly. Re-document these options as configuration options.

Additionally, add a new section documenting the format of the configuration
for each image type which uses it. In general, if configuration is used
directly (without a separate file) I have added documentation for it. If
the configuration points to a separate file, I have referenced that file's
documentation. Where there is no such documentation, I have added it.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>