doc: mkimage: Use standard style for synopsis

doc: mkimage: Use standard style for synopsis

The synopsis section is a bit messy. As an example, "uimage file name" is
printed in italics, bold, and roman (depending on the line). This cleans
things up and converts the synopsis section to use standard style. The
.SY/.YS macros set up appropriate formatting for command synopsis sections
(such as disabling hyphenation and setting a hanging indent). All parts of
the synopsis now use the following style:

- Bold for parts of the command which should be typed in by the user (such
  as the program name and flags)
- Italic for parts which should be replaced (such as uimage-file-name)
- Roman for parts which should not be typed at all (such as brackets)

Multi-word variables now use hyphens to connect their words instead of
spaces. This makes it clearer that all the words are part of the same
variable. Additionally, "option ..." is used to denote where other options
may be specified, as this appears to be standard style.

In addition to the above style changes, this also makes some changes to
content. The use of the term "legacy" has been removed, since this simply
refers to any non-FIT image type. Additionally, wording like "uimage file
name" has been replaced with "image-file-name" to better reflect that
images may or may not be uImages. Lastly, the "auto" value for -f is
documented in the synopsis.

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