doc: mkimage: Remove AUTHORS section

Per man-pages(7), "use of an AUTHORS section is strongly discouraged."
Remove it, and instead add some copyright notices and an SPDX. The default
license for U-Boot is GPL2, so that's what I put. The copyright dates are
based on the commit dates.

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