doc: mkimage: Reformat examples

doc: mkimage: Reformat examples

This puts each example in a new paragraph and uses a hanging indent for
continued lines to increase clarity. We use tabs instead of .in or .RS for
the indent because it renders properly in both man and mandoc (which is
what many common HTML man pages use). The only nit is that the tab stops in
man default to something like 2", so reduce that to 1". We also escape
every "minus" as recommended by man-pages(7).

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