binman: Add a --toolpath option to set the tool search path

binman: Add a --toolpath option to set the tool search path

Sometimes tools used by binman may not be in the normal PATH search path,
such as when the tool is built by the U-Boot build itself (e.g. mkimage).
Provide a way to specify an additional search path for tools. The flag
can be used multiple times.

Update the help to describe this option.

Signed-off-by: Simon Glass <sjg@chromium.org>