doc: mkimage: Use empty request instead of blank lines

doc: mkimage: Use empty request instead of blank lines

Blank lines do not have well-defined semantics in fill mode (the default).
Instead, use empty requests (.) where vertical space is necessary for
readability. There are a few places where we use a paragraph instead.

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