tools: dumpimage: Remove remaining mentions of the -i option

The -i option of the dumpimage tool has been removed so it should no
longer be documented in the README file. Refer readers to the tool's
help output rather than maintain a copy of the usage in the README.

Finally, adjust the example dumpfile invocation in imagetool.h to use
the -o option instead of the removed -i option.

Fixes: 12b83187 ("tools: dumpimage: Simplify arguments")
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Martyn Welch <martyn.welch@collabora.com>
Acked-by: Martyn Welch <martyn.welch@collabora.com>