doc: mkimage: Edit options for style and consistency

This makes a variety of changes for the options to make them
typographically consistent, clarify their meaning, and fix grammatical (or
other) errors. Many of the changes here are stylistic, though there are a
few fixes. The main changes I made across the board were:

- All options are bolded and parameters italicised
- All single quotes are properly matched (instead of using apostrophes)
- Minor background info has been added to clarify many underdocumented
  options
- Default values for options are documented

Signed-off-by: Sean Anderson <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

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