doc: usage: Add general rule for `$?`

For nearly all commands in U-Boot the '?' variable is handled the same
way with 0 meaning success, 1 meaning any failure.  Explain this in the
general rules section of the cmdline documentation (with a link to a
counter example) and then remove the redundant wording from most
commands. We retain a section about the return value in a number of
places where we are doing something such as always returning a specific
value or we have useful additional information to go along with the
normal return codes.

Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Heinrich Schuchardt <[email protected]>

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