tools: dumpimage: Clarify help

Help message isn't clear over the use of the "-T" option (it's to declare
the type of image that the tool is operating on), which also is optional
as it defaults to the default image type. It's also missing a description
of the "-o" option, so add it.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>